Using energy from the sun to heat water is one of the oldest uses of solar energy. Solar collectors are the heart of most solar energy systems. The collector absorbs the sun’s light energy and changes it into heat energy. This energy is than transferred to a fluid or air which are used to warm buildings, heat water, generate electricity, dry crops or cook food. Solar collectors can be used for nearly any process that requires heat.
Domestic hot water is the second-highest energy cost in the typical household in Europe or North America. In fact, for some homes it can be the highest energy expenditure. Solar water heating can reduce domestic water heating costs by as much as 70%. Designed to pre-heat the domestic water that is supplied to conventional water collector, it can result in remarkable savings. It’s easy to install and almost maintenance free.
Today, solar water heating systems are being used for single family houses, apartment buildings, schools, car washes, hospitals, restaurants, agricultural farms and different industries. This is a diverse list of private, commercial and industrial buildings, but they all have one thing in common – they all use hot water. Owners of these buildings have found that solar water heating systems are cost-effective in meeting their hot water needs all over the world.
Solar water heating was used long before fossil fuels dominated our energy system. The principles of solar heat have been known for thousands of years. A black surface gets hot in the sun, while a lighter coloured surface remains cooler, with white being the coolest. This principle is used by solar water collectors which are one of the best known applications for the direct use of the sun’s energy. They were developed some two hundred years ago and the first known flat plate collector was made by Swiss scientist Horace de Saussure in 1767, later used by Sir John Herschel to cook food during his South Africa expedition in the 1830’s.
Solar technology advanced to roughly it’s present design in 1908 when William J. Bailey of the Carnegie Steel Company (USA), invented a collector with an insulated box and copper coils. This collector was very similar to the thermosyphon system (described bellow). Bailey sold 4000 units by the end of World War I and a Florida businessperson who bought the patent rights sold nearly 60 000 units by 1941. In the U.S. the rationing of copper during World War II sent the solar water heating market into a sharp decline.
Little interest was shown in such devices until the world-wide oil crisis of 1973. This crisis promoted new interest in alternative energy sources. As a result, solar energy has, received increased attention and many countries are taking a keen interest in new developments. The efficiency of solar heating systems and collectors has improved from the early 1970s. The efficiencies can be attributed to the use of low-iron, tempered glass for glazing (low-iron glass allows the transmission of more solar energy than conventional glass), improved insulation, and the development of durable selective coatings.
SOLAR COLLECTOR MARKET
Solar domestic hot-water systems are technically mature and available practically all over the world. The market for flat-type collectors has been reported as substantial in Israel, China, Cyprus, Japan, Australia, Austria, Germany, Greece Turkey and USA. Sales in Europe are mainly for domestic water heating, which may also include space heating and heating swimming pools. World production of solar collectors in 1995 was 1,3 million m2 where market in Europe and Mediterranean countries is reported to be about 40% of the world market. Total amount of installed solar collectors exceeded 30 million m2 and the development of sales was very rapid since 1980. Since 1989 there is steady increase with around 20 % per year.
Among countries in Europe, Greece has become the leader in production of solar systems and exports 40% of all collectors produced and comprises 30% of the market in Germany. The industry‘s goal for the year 2005 represents 1,3 million systems and 5 million m2 of collectors. A project on Crete will need 20,000 collectors over two years. The Greek market installs 70,000 solar systems a year, reducing CO2 emissions by 1,5 million tonnes.
Sales in the EU in 1996 were reported to be over 0,7 million m2 of glazed collectors and about 0,15 million m2 of unglazed collectors (Renewable energy world, Sept. 1998). All the indications are that this trend will continue at a rapid pace since measures are being taken all over the EU for the promotion of solar systems.
Glazed solar collector production in 1994 (Source : Sun in action. The solar thermal market, a strategic plan for action in Europe. European Solar Industry Federation. Altener Program).
Country Production in 1994
Germany 170 000 m2
Greece 165 000 m2
Austria 100 000 m2
UK 40 000 m2
Denmark 20 000 m2
Others 55 000 m2
EU total 550 000 m2
Installed solar collector area in the world (Source: Sun in action. The solar thermal market, a strategic plan for action in Europe. European Solar Industry Federation. Altener Program).
Country Installed solar collector area
Mediterranean countries 8,5 million m2
USA 6,5 million m2
Japan 6 million m2
EU 5,6 million m2
Australia 2,5 million m2
China 1,5 million m2
Installed solar collector area per head of population was 0,5 m2 in Cyprus in 1992 the largest in Europe and followed by Greece and Austria. Collector area per head of population increased in Austria up to 0,2 m2 in 1998 and amounted total area of 1,5 million m2. Austria is first in sales per capita followed by Greece but both countries still fall behind the world leaders Israel and Cyprus. Analysis of statistical figures like collector area per head of population shows that favourable climatic conditions have less influence than socio-economic boundary conditions. The success in Cyprus is explained not only by the absence of any other local source of energy but also by countries regulation. Strong legislation promoting solar energy utilisation is in force also in Israel. Israel and Cyprus have imposed statutory requirements for solar heating systems in all new buildings. These requirements were introduced in stages: thus in Israel initially all new apartment buildings of up to eight storeys were required to have a community solar water heating system with appropriate storage tanks. This was later extended to all new dwellings in the country. Finally in 1983 new regulations required hotels, hospitals and schools to install solar water heating equipment. These regulations were coupled with financial incentives. A similar attempt has also been made in Cyprus and it was recently estimated that 90 % of individual dwellings and 15 % of apartments in Cyprus are now equipped with solar water heaters.
In Europe the total rapidly exploitable potential for solar collectors production is estimated to be 360 million m2 , representing a market volume of 50 billion USD at an annual average growth rate of 23%. In 2005 the area occupied by glazed solar collector installations in the EU is expected to rise to 28 million m2. Moreover, unglazed solar collectors for heating swimming pools are expected to reach 20 million m2.
SOLAR COLLECTORS TYPES
Typical solar collectors collect the sun’s energy usually with rooftop arrays of piping and net metal sheets, painted black to absorb as much radiation as possible. They are encased in glass or plastic and angled towards south to catch maximum sunshine. The collectors act as miniature greenhouses, trapping heat under their glass plates. Because solar radiation is so diffuse, the collectors must have a large area.
Solar collectors can be made in various sizes and constructions depending on requirements. They give enough hot water for washing, showers and cooking. They can be used also as pre-heaters for existing water heaters. Today there are several collectors on the market. They can be divided into several categories. One of them is division according temperature they produce:
Low-temperature collectors provide low grade heat, less than 50 degrees Celsius, through either metallic or non-metallic absorbers for applications such as swimming pool heating and low-grade water.
Medium-temperature collectors provide medium to high-grade heat (greater than 50 degrees Celsius, usually 60 to 80 degrees), either through glazed flat-plate collectors using air or liquid as the heat transfer medium or through concentrator collectors that concentrate the heat to levels greater than “one sun.” These include evacuated tube collectors, and are most commonly used for residential hot water heating.
High-temperature collectors are parabolic dish or trough collectors primarily used by independent power producers to generate electricity for the electric grid.
Batch Solar Water Collectors
The simplest type of solar water collector is a “batch” collector, so called because the collector is the storage tank – water is heated and stored a batch at a time. Batch collectors are used as pre-heaters for conventional or instantaneous water heaters. When hot water is used in the household, solar-preheated water is drawn into the conventional water collector. Since the water has already been heated by the sun, this reduces energy consumption. A batch solar water collector is a low cost alternative to an active solar hot water system, offering no moving parts, low maintenance, and zero operational cost. The acronym for a batch type solar water collector is ICS, meaning Integrated Collector and Storage. Batch collectors, also known as “breadbox” , use one or more black tanks filled with water and placed in an insulated, glazed box. Some boxes include reflectors to increase the solar radiation. Solar energy passes through the glazing and heats the water in the tanks. These devices are inexpensive solar water collectors but must be drained or protected from freezing when temperatures drop below freezing.
Flat-plate collectors are the most common collectors for residential water heating and space-heating installations. A typical flat-plate collector is an insulated metal box with a glass or plastic cover called the glazing and a dark-coloured absorber plate. The glazing can be transparent or translucent. Translucent (transmitting light only) low-iron glass is a common glazing material for flat-plate collectors because low-iron glass transmits a high percentage of the total available solar energy. The glazing allows the light to strike the absorber plate but reduces the amount of heat that can escape. The sides and bottom of the collector are usually insulated, further minimising heat loss.
The absorber plate is usually black because dark colours absorb more solar energy than light colours. Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar radiation into heat energy. The heat is transferred to the air or liquid passing through the flow tubes. Because most black paints still reflect approximately 10% of the incident radiation some absorber plates are covered with “selective coatings,” which retain the absorbed sunlight better and are more durable than ordinary black paint. The selective coating used in the collector consists of a very precise thin layer of an amorphous semiconductor plated on to a metal substratum. Selective coatings has both high absorptivity in the visible region and low emissivity in the long-wave infrared region.
Absorber plates are often made of metal usually copper or aluminium because they are both good heat conductors. Copper is more expensive, but is a better conductor and is less prone to corrosion than aluminium. An absorber plate must have high thermal conductivity, to transfer the collected energy to the water with minimum temperature loss. Flat-plate collectors fall into two basic categories: liquid and air. And both types can be either glazed or unglazed.
In a liquid collector, solar energy heats a liquid as it flows through tubes in the absorber plate. For this type of collector, the flow tubes are attached to the absorber plate so the heat absorbed by the absorber plate is readily conducted to the liquid.
The flow tubes can be routed in parallel, using inlet and outlet headers, or in a serpentine pattern. A serpentine pattern eliminates the possibility of header leaks and ensures uniform flow. A serpentine pattern can pose some problems for systems that must drain for freeze protection because the curved flow passages will not drain completely.
The simplest liquid systems use potable household water, which is heated as it passes directly through the collector and then flows to the house to be used for bathing, laundry, etc. This design is known as an “open-loop” (or “direct”) system. In areas where freezing temperatures are common, however, liquid collectors must either drain the water when the temperature drops or use an antifreeze type of heat-transfer fluid.
In systems with heat-transfer fluids, the transfer fluid absorbs heat from the collector and then passes through a heat exchanger. The heat exchanger, which generally is in the water storage tank inside the house, transfers heat to the water. Such designs are called “closed-loop” (or “indirect”) systems.
Glazed liquid collectors are used for heating household water and sometimes for space heating. Unglazed liquid collectors are commonly used to heat water for swimming pools. Because these collectors need not withstand high temperatures, they can use less expensive materials such as plastic or rubber. They also do not require freeze-proofing because swimming pools are generally used only in warm weather.
Air collectors have the advantage of eliminating the freezing and boiling problems associated with liquid systems. Although leaks are harder to detect and plug in an air system, they are also less troublesome than leaks in a liquid system. Air systems can often use less expensive materials, such as plastic glazing, because their operating temperatures are usually lower than those of liquid collectors.
Air collectors are simple, flat-plate collectors used primarily for space heating and drying crops. The absorber plates in air collectors can be metal sheets, layers of screen, or non-metallic materials. The air flows through the absorber by natural convection or when forced by a fan. Because air conducts heat much less readily than liquid does, less heat is transferred between the air and the absorber than in a liquid collector. In some solar air-heating systems, fans on the absorber are used to increase air turbulence and improve heat transfer. The disadvantage of this strategy is that it can also increase the amount of power needed for fans and, thus, increase the costs of operating the system. In colder climates, the air is routed between the absorber plate and the back insulation to reduce heat loss through the glazing. However, if the air will not be heated more than 17°C above the outdoor temperature, the air can flow on both sides of the absorber plate without sacrificing efficiency.
The best features of air collector systems are simplicity and reliability. The collectors are relatively simple devices. A well-made blower can be expected to have a 10 to 20 year life span if properly maintained, and the controls are extremely reliable. Since air will not freeze, no heat exchanger is required.
However, the use of solar air heating collectors is still limited to supply hot air for space heating and for drying of agricultural products mainly in developing countries. The major limitations for the wide adoption of solar air heaters are the high cost for commercially produced solar air heaters, the large collector area required due to the low density and the low specific heat capacity of the air compared to liquid heat transfer fluids, the extended air duct system required, the high power requirement for forcing the air through the collector, and the difficulty of heat storage. In countries with comparatively low insolation and extended periods of adverse weather, supplementary heat is required which increases investment costs to a level which limits its competitiveness to conventional heating systems. Promising ways to reduce the collector cost are the integration of the collector into the walls or roofs of buildings and the development of collectors which can be constructed using prefabricated components.
Heating with the solar wall .
Solar air heaters can be classified based on the mode of air circulation. In the bare plate collector, which is the most simple solar air heater, the air passes through the collector underneath the absorber. This kind of solar air heater is only suitable for temperature rise between 3 – 5 deg. Celsius due to the high convection and radiation losses at the surface. The top losses can be reduced significantly by covering the absorber with a transparent material of low transitivity for infrared radiation. The air flow occurs in this kind of solar air heater either underneath the absorber or between absorber and transparent cover. Due to the transparent cover, the incident radiation on the absorber is reduced slightly, but due to the reduction of the convective heat losses, temperature rise between 20 and 50 degrees Celsius can be achieved depending on insolation and air flow rate. A further reduction of the heat losses can be achieved if the air is made to pass above and underneath the absorber since this doubles the heat transfer area. The heat losses due to radiation will be reduced by this process due to lower absorber temperature. However, there is simultaneous reduction in the absorptivity of the absorber due to dust deposit if air flow is above or on both sides of the absorber.
Some solar air collectors eliminate the cost of the glazing, the metal box, and the insulation. Such a collector is made of black, perforated metal. The best heat transfer can be achieved by using porous material as absorber. The sun heats the metal, and a fan pulls air through the holes in the metal, which heats the air. For residential installations, these collectors are available in different sizes. Typical collector 2,4-meter by 0,8-meter panels are capable of heating 0,002 m3 per second of outside air. On a sunny winter day, the panel can produce temperatures up to 28°C higher than the outdoor air temperature. Transpired air collectors not only heat air, but also improve indoor air quality by directly preheating fresh outdoor air. These collectors have achieved very high efficiencies – more than 70% in some commercial applications. Plus, because the collectors require no glazing or insulation, they are inexpensive to manufacture.
Conventional simple flat-plate solar collectors were developed for use in sunny and warm climates. Their benefits are greatly reduced when conditions become unfavourable during cold, cloudy and windy days. Furthermore, weathering influences such as condensation and moisture will cause early deterioration of internal materials resulting in reduced performance and system failure. These shortcomings are reduced in evacuated-tube collectors.
Evacuated-tube collectors heat water in residential applications that require higher temperatures. In an evacuated-tube collector, sunlight enters through the outer glass tube, strikes the absorber tube, and changes to heat. The heat is transferred to the liquid flowing through the absorber tube. The collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate in a flat-plate collector) covered with a selective coating. The heated liquid circulates through heat exchanger and gives off its heat to water that is stored in a solar storage tank.
Evacuated tube collectors are modular tubes which can be added or removed as hot-water needs change. When evacuated tubes are manufactured, air is evacuated from the space between the two tubes, forming a vacuum. Conductive and convective heat losses are eliminated because there is no air to conduct heat or to circulate and cause convective losses. There can still be some radiant heat loss (heat energy will move through space from a warmer to a cooler surface, even across a vacuum). However, this loss is small and of little importance compared with the amount of heat transferred to the liquid in the absorber tube. The vacuum in the glass tube, being the best possible insulation for a solar collector, suppresses heat losses and also protects the absorber plate and the “heat-pipe” from external adverse conditions. This results in exceptional performance far superior to any other type of solar collector.
Evacuated-tube collectors are available in a number of designs. Some use a third glass tube inside the absorber tube or other configurations of heat-transfer fins and fluid tubes. One commercially available evacuated-tube collector stores 19 litres of water in each tube, eliminating the need for a separate solar storage tank. Reflectors placed behind the evacuated tubes can help to focus additional sunlight on the collector.
Due to the atmospheric pressure and the technical problems related to the sealing of the collector casing, the construction of an evacuated flat-plate collector is extremely difficult. To overcome the enormous atmospheric pressure, many internal supports for the transparent cover pane must be introduced. However, the problems of an effective high vacuum system with reasonable production costs remain so far unsolved. It is more feasible to apply and adapt the mature technology related to the lamp industries with proven mass production. Building a tubular evacuated solar collector and the maintenance of its high vacuum, similar to light bulbs and TV tubes, is practical. The ideal vacuum insulation of the tubular evacuated solar collector, obtained by means of a suitable exhausting process, has to be maintained during the life of the device to reduce the thermal losses through the internal gaseous atmosphere (convection losses).
In high temperature region these collectors are more efficient than flat-plate collectors for a couple of reasons. First, they perform well in both direct and diffuse solar radiation. This characteristic, combined with the fact that the vacuum minimizes heat losses to the outdoors, makes these collectors particularly useful in areas with cold, cloudy winters. Second, because of the circular shape of the evacuated tube, sunlight is perpendicular to the absorber for most of the day. For comparison, in a flat-plate collector that is in a fixed position, the sun is only perpendicular to the collector at noon. Evacuated-tube collectors achieve both higher temperatures and higher efficiencies than flat-plate collectors, but they are also more expensive.
Concentrating collectors use mirrored surfaces to concentrate the sun’s energy on an absorber called a receiver. They also achieve higher temperatures than flat-plate collectors, however concentrators can only focus direct solar radiation, with the result being that their performance is poor on hazy or cloudy days. The mirrored surface focuses sunlight collected over a large area onto a smaller absorber area to achieve high temperatures. Some designs concentrate solar energy onto a focal point, while others concentrate the sun’s rays along a thin line called the focal line. The receiver is located at the focal point or along the focal line. A heat-transfer fluid flows through the receiver and absorbs heat. Concentrators are most practical in areas of high insolation, such as those close to the equator and in the desert areas.
Concentrators perform best when pointed directly at the sun. To do this, these systems use tracking mechanisms to move the collectors during the day to keep them focused on the sun. Single-axis trackers move east to west; dual-axis trackers move east and west and north and south (to follow the sun throughout the year). Concentrators are used mostly in commercial applications because they are expensive and because the trackers need frequent maintenance. Some residential solar energy systems use parabolic-trough concentrating systems. These installations can provide hot water, space heating, and water purification. Most residential systems use single-axis trackers, which are less expensive and simpler than dual-axis trackers. For more information about concentrating collectors see chapter Solar Thermal Power Production.
SOLAR COOKERS AND STILLS
There exists also some other inexpensive, “low-tech” solar collectors with specific functions like solar box cookers (used for cooking) and solar stills producing inexpensive distilled water from virtually any water source.
Solar box cookers are inexpensive to buy and easy to build and use. They consist of a roomy, insulated box lined with reflective material, covered with glazing, and fitted with an external reflector. Black cooking pots serve as absorbers, heating up more quickly than aluminium or stainless steel cookware. Box cookers can also be used to kill bacteria in water if the temperature can reach the boiling point.
Solar stills (see chapter on Solar water distillation) provide inexpensive distilled water from even salty or badly contaminated water. They work on the principle that water in an open container will evaporate. A solar still uses solar energy to speed up the evaporation process. The stills consist of an insulated, dark-coloured container covered with glazing that is tilted so the condensing fresh water can trickle into a collection trough. A small solar still, which is about the size of kitchen stove, can produce up to ten litres of distilled water on a sunny day.
Solar energy has a variety of practical and cost-effective applications in today’s homes and buildings. The main applications of solar collectors are as follows :
hot water preparation in households, commercial buildings and industry,
water heating in swimming pools,
space heating in buildings,
drying crops and houses,
space cooling and refrigeration,
The technologies for all applications are considered to be mature and for the first two, under the appropriate conditions, economically viable. Separate chapter is devoted to concentrating collectors which are cost effectively used for power production especially in regions with high insolation (see chapter on Solar Thermal Power).
Solar Thermal Residential Water Heating
Today, several million homes and businesses use solar water heating systems. These systems are providing consumers a cost-effective and reliable choice for hot water. Taking a shower with solar-heated water, or heating a house with solar-heated air or water, is a natural and simple method for both conserving energy and saving fossil fuels. When a solar heating system has been designed and installed correctly, it can be aesthetically appealing and also add to the value of the home. On new construction, they can be worked into the building design to be almost invisible, while on existing construction it can be a real challenge to make them fit in.
A solar water collector is saving an owner money but it also help protect the environment. Emissions of one to two tons of carbon dioxide are saved by a single conventional water collector every year. Other pollutants, such as sulphur dioxides, carbon monoxide and nitrous oxides are also displaced when a homeowner decides to tap into a solar energy.
Hot water production is the most widely distributed utilisation of direct solar heating. An installation consists of one or more collectors in which a fluid is heated by the sun, plus a hot-water tank where the water is heated by the hot liquid. Even in the areas of low insolation like in Northern Europe a solar heating system can provide 50-70% of the hot water demand. It is not possible to obtain more, unless there is a seasonal storage (see chapter below). In Southern Europe a solar collector is able to cover 70-90% of the hot-water consumption. Heating water with the sun is very practical and cost effective. While photovoltaics (see chapter on photovoltaics) range from 10-15% efficiency, thermal water panels range from 50-90% efficiency. In combination with a wood stove coil/loop, virtually year round domestic hot water can be obtained without the use of fossil fuels.
HOW IS A SOLAR WATER COLLECTOR COMPETITIVE WITH CONVENTIONAL HEATERS ?
Costs of complete solar water heating systems differs considerably from country to country (in Europe and the USA e.g. between 2000 – 4000 USD). They also depend on hot water requirements and the climate conditions in the area. This is usually a higher initial investment than required for an electric or gas heater but when adding all of the costs involved with heating water in home, the life-cycle cost of a solar water heating system is usually lower than traditional heating system. It must be noted that simple pay-back time for investment into solar heating system depends on prices of fossil fuels substituted by solar energy. In EU countries pay-back times are generally less than 10 years. The expected life span of the solar heating system is 20-30 years.
Important feature of solar installation is energy pay-back time – time needed to produce as much energy by solar system as it was needed to produce this system. In Northern Europe with less solar radiation than in other parts of the world a solar heating system for hot-water preparation has an energy pay back period of 3-4 years.
HOW MUCH ENERGY CAN WE GET ?
The amount of energy we can get from solar heating system depends on available insolation and efficiency of the solar system. Insolation differs widely in the world and is crucial for solar system. The amount of solar radiation available in some regions of the world is given in chapter Solar Radiation. The efficiency of solar system depends on efficiency of solar collector and losses in the hot water circulation system. As the later depends on various specific parameters we will focus only on solar collector efficiency. Efficiency is defined as the ration between the amount of energy produced and solar energy falling down on collector. Efficiencies are different for different collector types and depends on solar intensity, thermal and optical losses – higher losses means lower efficiencies. Thermal losses are minimal if the temperature of water used for application is the same as ambient air temperature. Thus simple absorber without glazing used for pool heating achieve the highest efficiencies up to 90%. But when these collectors are used for warm domestic hot water preparation (water temperature 40 degrees Celsius higher than ambient air temperature) their efficiencies are usually lower than 20%. In this case the best results are achieved by flat-plate collectors (with selective coatings) and evacuated tube collectors which are best suited for this application. When higher water temperatures are needed (e.g. for space heating) evacuated -tube collectors are the best but also the most expensive.
Solar collector efficiencies for insolation typical for Central Europe at noon
during summer day – 800 W/m2. Efficiency at temperature difference (*)
Collector Type 0 deg. C
pool heating 40 deg. C
domestic hot water 50 deg. C (**)
Absorber without glazing 90 % 20 % 0 %
Flat-plate (non-selective coating) 75 % 35 % 0 %
Flat-plate (selective coating) 80 % 55 % 25 %
Evacuated-tube 60 % 55 % 50 %
* Difference between ambient temperature and temperature of water inside solar collector.
** Values are related to lower insolation during early spring (400 W/m2).
Low efficiency of evacuated tube collector in low temperature region is caused by high optical losses on curved surface of the glass.
Bearing in mind that there are huge differences between prices of collectors it is obvious that the crucial criteria for collector type selection is purpose of its utilisation. A comparison of different collector types and their economy features are given in the table below.
Typical characteristics of different types of solar collectors according German ministry of economy are following.
Purpose Collector type Temp. in deg.C Production kWh/m2/year Price in DM DM/kWh (*)
Pool heating Absorber 20-40 250-300 100-250 0,02-0,04
Warm water preparation Flat-plate
Drying Air collector 20-50 300-400 400-1000 0,06-0,13
* per m2 under 20 years collector life expectation.
Guidelines on Solar Water Heating System Sizing
A solar water heating system can be used as the sole source for hot water or may include a back-up conventional system to meet heavy or unusual hot water requirements throughout the year. Systems are usually sized according to the number of rooms, people and household water needs. There are several different configurations of solar water heating systems. In general, however, there are two main types: active systems which have pumps and controls to deliver solar heat to the storage tank, and passive systems like thermosiphons which utilise natural circulation of hot water.
When designing a solar water heating system, it is important to decide first how much hot water will be used per average day. If the amount of hot water is known, the size of system (collectors, storage tank) have to be calculated. Here are some general remarks on what should be taken into consideration when designing solar heating system.
The main part of the solar heating system are the solar collectors. Most frequently used are flat-plate collectors consisting of an absorber where the solar radiation is transferred to heat in the solar collector fluid, insulation along the edge and under the absorber a case that holds everything together, and allows the necessary ventilation and a glass or plastic cover.
When glass is used as cover, it is important that the iron content is low or zero, so at least 95% of the solar radiation pass through the glass. In practice no more than single layer of glass is used. If a plastic cover is used, it is important that the plastic can stand up to the UV-rays from the sun. It has been found that polycarbonate plates are very satisfactory.
The absorber can be made of a plate with tubes where the collector fluid flows. Usually the absorber is made of copper or stainless steel. Experience have shown, that best absorber tubes are those made from copper. Ordinary steel tubes cause big problems with corrosion. It is essential that the absorber can stand up to the UV-light from the sun, and the stagnation temperature (dry-boiling temperature), which is 100-140 deg.C for solar collectors without selective coating, and 150-200 deg.C with selective coating.
Construction of a flat plate collector requires soldering and brazing of tubes and physically bonding the tubes to sheet. The more physical contact between the sheet and the tubes, the more heat transfer to the fluid moving through the tubes. The absorber is often covered by a selective black coating, which absorbs the sun rays, but holds back the heat radiation. The problem with normal black paint is that it will outgas, or boil off the metal under the extreme heat. Also, under normal cases, black paint will radiate heat, rather than absorb it for transfer to the fluid.
Many choices for the framework of solar collectors are reasonably available. Wood, plastic, steel or aluminium have all been used with varying degrees of success, but nothing is as good as aluminium. Aluminium weathers the elements with very low maintenance, and has colour choices baked on, so there is no need to paint the exterior of solar panel. Over the years, plastics have proven to be a poor choice for the major parts of a solar panel. For the exterior, plastic has a nasty habit of degrading from the sun’s ultraviolet rays. Plastic discolours and eventually becomes brittle and cracks. Plastic also has a high coefficient of expansion. This means it expands and contracts so much that making the joints weather tight is difficult. Using steel for framework means also some problems. One is that the panels need painting regularly and two, they react chemically with the copper interior.
Solar collectors are usually mounted directly on top of the roof, or at a frame placed on a flat roof or the ground. Solar collectors can also be integrated in the roofing. In some cases problems with sealing between the solar collector and the rest of the roof can arise.
The size of solar collectors depends on the daily hot water requirements. In general one person may require approx. up to 50 litres of hot water at approx. 55° to 60° degrees Celsius per day (for domestic bathing only, without laundry). It has been shown that in average 1-1,5 m2 solar collector area is needed per 50 litres daily consumption of hot water. Selection of size would also depend on availability of standard products. Prizes vary with the collector size and with the installation charges. Installation is simplest when the system is incorporated in the initial planning of the construction of a new house. This allows the architect to incorporate the collectors into the plan, both esthetically and economically.
SOLAR COLLECTOR ORIENTATION
The orientation of solar collectors (which way they face and how they are tilted) optimizes their collection ability. The earth’s atmosphere absorbs and reflects a significant portion of solar radiation. Thus, the most energy that can be gathered on any given sunny day is at solar noon, when the direct beam radiation is least affected by the atmosphere. Solar noon is true south in the northern hemisphere. Although orienting the collectors to true south will normally maximize performance, a variation within 20° east or west is acceptable without additional collector surface area.
A solar collector that traces the sun, will usually receive about 20% more solar radiation than a south facing optimum placed collector. This additional output do not compensate the costs related to a construction, which has to trace the sun. Usually it will be cheaper to install a 20% larger solar collector.
Local weather patterns (i.e., morning haze or prevailing afternoon cloudiness) should also be considered in collector orientation. If local weather is not a factor and collectors cannot be faced true south, orienting them to the west is generally preferable due to higher afternoon temperatures (collectors have less heat loss with higher outside temperatures).
Since elevation of the sun varies throughout the year depending on local latitude, collectors should be tilted towards the sun depending upon application. In general, seasonal differences in irradiation are considerable and must be taken into account for all solar energy applications. Tilting the collecting surface some 30…50 degrees to the South in the Northern Hemisphere or to the North in the Southern Hemisphere yields somewhat better wintertime results for the region in question, but also some losses in summer. Space heating systems are tilted more to the position of the winter sun. In the tropics, a nearly horizontal receiving surface is generally most advantageous because of the sun’s high altitude. The most desired angle of inclination to mount the solar collector is the local latitude. Positive difference between latitude and roof angle results better system performance in winter. Lower solar collector mounting angle than the local latitude will result in greater system performance in summer. Variations of solar collector tilt angle for architectural reasons can be compensated with additional collector size.
The storage tank shall store the solar heat. This is done by storing hot water until it is needed. There are several different sizes of tanks available. All tanks must have connections for cold water inlet and hot water outlet as well as two connections for circulation pipes. Hot water storage tanks can easily be fitted to a stand. The most efficient is a vertical tank with good temperature stratification, so the cold inlet water aren’t mixed with the warmer water at the top of the tank. A horizontal tank reduces the output by 10-20%.
The heat from the solar collectors is delivered to the water in a heat exchanger. As heat exchanger is mostly used a coil in the bottom of the tank, or a cap around the tank with collector fluid. In low-flow and self-circulating systems a cap are always used. In low-flow systems the solar collector fluid flows slowly down through the cap of the storage tank, which gives a stratification of collector fluid in the cap corresponding to the stratification in the tank. This gives more ideal heat transfer, and thereby a higher efficiency than in traditional systems.
All hot water storage tanks must be well insulated to keep the water hot during the night. Heat loss depends on many factors (ambient temperature, wind, season, etc.) and will be approximately 0,5 to 1 degree Celsius per hour during the night. The insulation of the tank must be so good, that hot water from a sunny day still is hot two days later. Especially the top must be well insulated, and without thermal bridges. Experience shows that a minimum thickness of insulation of 100 mm should be maintained.
It must be ensured that piping from the storage tank do not lead to self-circulation, which can drain the tank for hot water during periods without hot water consumption. If there is a flow tube pipe for the hot water, this must not be connected to the cold water; but has to enter at the upper part of the tank. Usually the outlet of the storage tank is equipped with a scalding protection, so the water delivered for use never gets warmer than e.g. 60 deg.C, regardless of the temperature in the tank.
The solar water collector storage tank should have a size of 80 litres of hot water storage volume per person with a hot water consumption of 50 litres per day. These are the average values. If the home have a dishwasher, washing machine, several children taking daily showers or baths during the day, so all of this water usage must be figured into the total water needs.
Solar Collector Circuit
The solar collector circuit connects the solar collector to the storage tank. The components of the circuit are:
a pump that ensures circulation (not needed in self-circulating systems). The pump is usually controlled by a difference thermostat, so it starts running, when the solar collector is a bit warmer than the storage tank. If the storage tank has a heat exchanger coil at the bottom, a more simple control system can be used; e.g. a light sensor, or a timer that starts the pump during day time.
pipelines connecting hot water storage tank and collectors. Layout of pipelines should secure to be of shortest possible distance. Pipes should not be exposed to the weather if possible. Best is to keep them inside the house where possible. It is important to have several separate pipes from the collector to the taps to reduce heat losses (smaller pipes) and to give a fast supply of hot water to the user, with a maximum delay of about 10 to 20 seconds. Pipelines must be produced of a non-corroding material. Systems with open expansion are most risky to get corrosion problems.
a one-way valve which prevents that the solar collector fluid runs backwards at night, and empties the storage tank for heat (not necessary in all kinds of installations).
an expansion tank; either an open container at the top of the installation, or a pressurised expansion tank that contains minimum 5% of the solar collector fluid.
overpressure protection (only in connection with pressurized expansion tank); must be a type that manage to let out the solar collector fluid, if the system is boiling. There must always be an accumulation tank to the fluid in case of boiling. This is normally a safety valve and a non-return valve (check), or a non-return valve and a vent pipe which will release over-pressure due to the increase of volume by heating.
air outlets, automatic or simply screws; must be used at all height points in the system, as air pockets always will appear.
dirt filter for the pump, to remove dirt, e.g. from the installation (can be spared in some installations).
manometers and thermometers according to need.
the solar collector fluid must be able to stand frost, and must not be toxic.
Usually is used an approved liquid, consisting of water with 40% propylene glycol (can stand minus 20 deg.C), and a substance that can be seen and tasted, if solar collector fluid leak to the tap water. Oil can also be used as collector fluid, but it is difficult to make a collector circuit with oil tight.
The simplicity of solar water heating systems means that maintenance is minimal. Required maintenance will depend on type of system. Experience shows that once or twice a year it must be controlled, that there are enough fluid and pressure on the system. Once a year it should be checked that the solar collector fluid hasn’t become acid. Acid indicator paper can be used. Acid fluid should be changed. In case the system is boiling, it is simply needed to fill new fluid on the system; as the old fluid may be damaged by the boiling.
An important consideration when designing a system is the freeze-protection requirements. Some storage tanks must be softened, and the anti-corrosion zinc block shall be changed after approximately 10 years, it prolongs the life span significantly.
GUIDELINES FOR SOLAR COLLECTOR SYSTEM SIZING
For a typical solar water collectors (heating from 8 to 45 deg.C) with selective absorbers, the following hand rules can be used:
in average 50 litres of hot water per person and day is needed.
1-1,5 m2 solar collector area is needed per 50 litres daily consumption of hot water.
the storage tank shall be 40-70 litres per m2 solar collector or 80 litres per head.
the heat exchanger in the storage tank shall be able to transfer 40-60 W/deg.C per m2 solar collector at 50 deg.C.
If these guidelines are followed, a typical solar water collector installed in Northern Europe will cover 60-70% of the annual hot water consumption, and be able to produce 350-500 kWh/m2 per year. For larger buildings (e.g. hotels, hospitals, apartment blocks), the collector areas and storage volumes required per head are smaller, but good dimensioning needs detailed analysis of demand and local climate conditions. The experience shows that solar systems for hot water preparation should be designed to be as simple as possible and not oversized.
For a family with 4 persons which uses 200 litre of hot water each day solar collector with 6 m2 area are needed. During the year they can produce up to 3000 kWh of clean energy. When solar collectors substitute the oil boiler than net saving can achieve at least 300 litres of oil annually.
Thermosiphons are solar water heating systems with natural circulation (i.e. by convection) which can be used in non-freezing areas. These systems are not the highest in overall efficiency but they do offer many advantages to the home builder. They are simple to make and most of these devices operate without the assistance of an electric pump. This thermosiphon circulation occurs because of the variation of water density with its temperature. With the heating of the water in the collector (usually flat-plate), the warm water rises, and since it is connected in a riser pipe to the hot water storage tank and a down-comer pipe again to the collector, it is replaced by the cooler, heavier cold water from the bottom of the hot water storage tank. It is therefore necessary to place the collectors below the hot water storage tank and to insulate both connecting circulation pipes.
Thermosiphon systems have serious problems with their collectors freezing and bursting, even in areas with only one or two mild freezes a year. It only takes one frozen night to ruin an unprotected collector. Some systems are designed to avoid freeze damage by using 10 centimetres or larger copper tubing in a double glazed, insulated enclosure. Quite simply, the volume of water in system is too large to freeze and burst in a mild freeze. This type of installations is popular in sub-tropical and tropical areas.
The complete thermosiphon circulation system may be divided into three separate sections:
The flat plate collector (absorber).
The circulation piping.
The hot water storage tank (boiler).
Usually solar collector is located on a lower story, porch, or shed roof so that the top of the panel is at least 50 centimetres below the bottom of the storage tank. Tank location is usually in a second story, an attic, sometimes a cupola – somewhere that ensures an 50 cm vertical height difference between panel and the tank.
Solar Pool Heating
Solar pool heating system is a wise investment. In the USA the Department of Energy has identified swimming pools as a huge consumer of energy across the country, and has recognized pool heating as one of the most cost-effective means of reducing energy consumption. Solar pool heating systems are being used in virtually every area of the United States or Europe. Over 200 000 pools are heated by solar in the United States alone. The oldest systems have been in use for more than 25 years, and are cost-effective, highly reliable and require minimal maintenance. Important fact is that they function well and are cost-effective for the swimming season even in northern climates. Systems can also be designed for indoor pools as well as for larger municipal and commercial pools.
Despite the fact that price of installation varies on the size of the pool and other site-specific installation conditions if solar systems are installed in order to reduce or eliminate fuel or electricity consumption, they generally pay for themselves in energy savings in many countries in two to four years. Moreover solar pool heating can extend the swimming season by several weeks without additional cost.
Most homes can accommodate a solar pool heating system. These systems can be as simple as water running through a black hose. For outside pools, the only thing which is needed is the absorber portion of the solar collector. Inside pools need standard solar collectors to provide winter heating.
Although solar collectors are often installed on a roof, they can be installed wherever they can be exposed to the sun for a good portion of the day. The type of roof or roofing material is not important. The appropriate area of solar collectors required for a given swimming pool is directly related to the area of the pool itself. The proper ratio of pool area to solar collector area will vary according to such factors as location, the orientation of the solar collectors, the amount of shading on the pool or solar collectors, and the desired swimming season. In general, however, the area of solar collectors required is usually 50% to 100% of the pool surface area.
HOW DO SOLAR POOL HEATING SYSTEMS WORK?
Adequate swimming pool heating can be achieved by having low temperature collectors directly connected to the filter circulation. In a few cases an additional “booster pump” or a slightly larger filtration pump may be needed. Today’s most efficient systems employ the use of an automatically controlled diverting valve. The pool’s filtration system is set to run during the period of most intense sunshine. During this period, when the solar control senses that adequate heat is present in the solar collectors, it causes a motorized diverting valve to turn, forcing the flow of pool water through the solar collectors, where water is heated. The heated water then returns to the pool. When heat is no longer present, the water bypasses the solar collector. Thus, most systems have very few moving parts which minimizes operation and maintenance requirements. Additional precautions are required against corrosion in collectors, since the water is quite aggressive (use of low temperature collectors, possibly made of plastics).
PLACING THE SYSTEMS
Systems can quite easily be placed out of sight in a remote places, for example upon a suitable roof; however some basic design rules should be observed. The chosen site should be level or slightly sloping (less than 30 deg. to horizontal) with the return manifolds higher than the infeed manifolds and all hoses rising steadily from one to the other to ensure all air is expelled during operation.
Both a non-return valve and a vacuum release valve should be fitted to systems placed at more than 1 meter above pool level to prevent the reverse flow of water into the pool and the flattening of hoses when the collector drains at the end of each operating cycle. All connections into the pool filtration circuit must be made after the filter unit and, if applicable, before any existing conventional heater to avoid pressurising the solar system.
OPERATION AND MAINTENANCE
The simplicity of solar pool heating systems means that operation and maintenance requirements are minimal. In fact, in most cases no additional maintenance beyond normal filter cleaning and winter close-up is necessary. The system should be drained in the winter months; however, in some cases even this may not be necessary because the system drains itself. In addition, solar pool heating equipment is so reliable that many solar pool collector manufacturers provide warranty coverage for their products which far exceeds that of automobiles and household appliances.
SOLAR SPACE HEATING
So far only systems for warm water preparation have been described. An active solar heating plant can provide hot water, and additional heating via the central heating system at the same time. To get a reasonable output, the central heating temperature must be as low as possible (preferably around 50 deg.C), and there must be a storage for the space heating. A smart solution is to combine the solar heating installation with under-floor heating, where the floor function as heat storage.
Solar heating installations for space heating usually give less profit than hot-water installations, both according economy and energy, as heating is seldom needed during summer. But if heat is needed during summer (like in some mountain areas), then space heating installations is a good idea. In central Europe, some 20% of the total heat load of a traditional house, and close to 50% low energy house, could be supplied by an advanced active solar heating system employing water storage only. The remaining heat need to be drawn from auxiliary energy systems. To increase the solar fraction, would in practice require larger storage capacities.
For single houses, systems with well-insulated water tanks between 5-30 m³ have been constructed especially in Switzerland (so-called Jenni system) but the costs are too high and the storage is often unpractical. The solar fraction of a Jenni-system is >50% and may reach even 100%.
If all of the load in the above example were supplied by an up-to-date active solar heating system, a 25 m² collector area and 85 m³ storage water tank with 100 cm insulation around would be needed. Improving the energy storage capacity of the storage unit, would dramatically improve the practical possibilities for storage.
Although individual solar space heating is technically feasible, it is likely that it would be far more cost effective to invest in insulation to cut space heating demands.
If a far larger collector together with a much larger storage tank were fitted, solar energy should be able to supply energy for several houses. Basic problem with solar energy is related to the fact that most of the energy is needed during the winter when solar insolation is the lowest and on the other side much of summer potential output can not be used because the demand is mostly not there. So capital investment into larger collectors with larger gains would be wasted.
Despite this fact there are several installations using summer heat produced by solar collectors and saved through to the winter. These installations are using large storage tanks (seasonal storage). Problem is that the volume of hot water storage needed to supply a house is almost the same size as the house itself. In addition, the tank would need to be better insulated. A normal domestic hot water cylinder would require insulation of 4 metres thick to retain most of its heat from summer to winter. It therefore pays to make storage volume really enormous. This reduces the ratio of surface area to volume.
Large solar heating plants for district heating are now in use, e.g. in Denmark, Sweden, Switzerland, France or USA. Solar modules are mostly installed directly at the ground in larger fields. Without a storage such solar heating installation would cover approximately 5% of the annual heat demand, as the plant never must produce more than the minimum heat consumption, including loss in the district heating system (by 20% transmission loss). If there is a day-to-night storage, then the solar heating installation can cover 10-12% of the heat demand including transmission loss, and with a seasonal storage up to 100%. There is also a possibility to combine district heating with individual solar water collectors. Then the district heating system can be closed during summer, when the sun provides hot water, and there is no need for space heating.
PRESENT SOLAR STORAGE SYSTEMS
Large-size seasonal storage systems for communities have been demonstrated in several countries but are still too expensive. The size of a central storage system may range from a few thousand m3 up to a few 100 000 m3. The largest storage project in Europe is in Oulu, Finland where a large rock cavern heat storage of 200 000 m3 will be connected to a combined heat and power plant burning biomass. This district heating plant was built under the EU-Thermie programme.
Another successful project with seasonal storage of hot water has been constructed in Lyckebo, Sweden. This project is using a rock cavern filled with water (volume of 105 000 m3) and flat plate solar collectors with area of 28 800 m2 which supply 100% energy (8500 MWh/a) for space and water heating of 550 dwellings. All houses are connected to communal district heating system. The temperature of supply water is 70 degrees Celsius and the temperature of return water is 55 degrees.
The pay-back times of such installations are very long. The important lesson from space heating systems has been that it is essential to invest in energy conservation and passive solar design first and then to use solar energy to help supply the remaining reduced load.
COMBINING SOLAR WITH OTHER RENEWABLE SOURCES
Combining renewable energy sources such as solar heat with solar storage in form of biomass may be a good solution. Or, if the remaining load of a low energy house is very low, some liquid or gaseous biofuels with advanced burners together with solar heating may be used.
Solar heating together with solid biomass boilers may provide interesting synergy and also solution to the seasonal storage of solar energy. Using biomass in the summer may be non-optimal, as the boiler efficiencies at partial loads are low and also relative piping losses may be high – in smaller systems using wood in the summer may even be uncomfortable. Solar heating may well provide 100% of the summertime loads in such cases. In the winter, when the solar yield is negligible, the biomass options provides almost all of the heat needed.
Experiences notably from central Europe with solar heating and biomass together are positive. Some 20-30% of the total load is typically provided by solar heating and the main load, i.e. 70-80% of the total load, by biomass. Combined solar heat and biomass may be used for both single-family houses and for district heating. For central European conditions, around 10 m³ of biomass (e.g. wood) would be enough for a single-family house with solar heating system replacing well up to 3 m³ per year in a household.
Solar Thermal Commercial Water Heating
Many businesses use solar water heating to preheat the water before using another method to heat it to boiling or for steam. Being less dependent on fluctuating fuel prices is another factor that makes solar system a wise investment. In many cases installation of solar water heating will derive an immediate and significant savings in energy costs. Depending on the volume of hot water needed and the local climate a business can realize savings of 40 – 80% on electric or fuel bills. For example the 24-story Kook Jae office building in Seoul, South Korea meets over 85% of its daily hot water needs with a solar hot water heating system. The system has been in operation since 1984 and is so efficient that it has exceeded it’s design specifications and even provides 10 to 20 percent of the annual space heating requirement.
Solar heating at Kook Jae building.
There are several different configurations of solar water heating systems. In general, however, the amount of hot water that a commercial business demands requires an active system. Active systems typically consist of solar collectors on a south-facing roof (in Northern hemisphere), and a storage tank near the existing water collector. When sufficient heat is present in the solar panel, a “controller” turns on a pump which begins circulating fluid, either water or antifreeze, through the solar panel. The fluid picks up the heat from the collector and transfers the heat to the potable water supply which is stored in a tank until needed. If the solar-heated water is not at the desired temperature, a back-up energy source can be used to bring the water temperature up to the desired level. The type and size of a system is calculated by determining ‘ water-heating load similar to the way described in chapter on solar collector sizing for households (see above). Similarly required maintenance for commercial systems will depend on the type and size of system, but the simplicity of solar water heating systems means that maintenance is minimal.
While for many businesses the biggest advantage of a solar water collector is the resulting savings in utility bills, value must be placed on the substantial environmental benefit. Air pollutants, such as sulphur dioxides, carbon monoxide and nitrous oxides are also displaced when a business owner decides to tap into a cleaner source of energy – the sun.
Industrial Process Heat
Industry requires heat in a variety of temperature ranges, depending on the process at hand. Many of these processes can be served by collectors ranging from the flat-plate variety, which are restricted to temperatures below 100 degrees C, to concentrating collectors which can produce temperatures of several hundred degrees.