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Converting solar energy into heat requires the utilization of solar thermal collectors. Solar thermal collectors can be classified based on the heat requirement into:

Non-concentrating collectors fully utilize the global radiation. The simplest design of a non-concentrating collector is the flat plate collector. 

The absorbers are black painted metal-either copper, aluminum, steel or plastic plates. In order to reduce the useful heat losses-which increase with rising temperatures-transparent covers are placed on the collectors and the heat losses at the back of the absorber are reduced by appropriate insulation. With these collector temperatures up to 80°C with conversion efficiency of about 50-60% can be achieved. The properties of this collector are well known and they are manufactured in many parts of the world.
Applications
Solar thermal systems are mostly used in residential and industrial applications such as domestic water heating, heating of swimming pools, space heating, water processes for industrial heating and agricultural drying. These products are reliable and show a high technical standard for low temperature demand.

Concentrating collectors use mainly the direct beam of the radiation by concentrating irradiation on the absorber thus increasing the intensity of radiation on the absorber. The concentrator uses a mirror or a lens to concentrate light. Concentrating collector systems are preferred technology in regions with more than 2,500 annual sunshine hours to obtain fluid temperatures above 150°C. The concentrator is normally equipped with a tracking device that follows the sun with the absorber located along the focus to extract the maximum amount of sunlight for heating. There are two types of concentrators:
• Linear focusing
• Point focusing concentrator
Applications
Desalination plants, solar cookers.

• Has been commercially available for about 30 years. These systems can reduce the heating and cooling load by 50% with no additional cost and some systems can reach 75% heating and cooling load reduction with modest additional cost.

• The majority of the energy used by commercial and industrial companies is below 250°C, a temperature range perfect for solar technologies. Solar collectors used in industrial and commercial processes, such as cleaning, drying, sterilization and pasteurization, heating of productions halls, can reach energy savings of 75% to 80% with payback periods under five years.

• One of the most promising agricultural applications for active solar heating worldwide is the drying of agricultural products. Wood and conventional fossil fuels are used extensively, and in many countries more expensive diesel and propane fuels are replacing wood. While solar crop drying is commercially available for specific crops in specific locations, its market share is insignificant at this time.

• Over 75% of the energy used in single and multi-family homes is for space heating and hot water preparation. Solar energy can meet, with existing technologies, up to 70-80% of this heating demand depending on the climate.

• Office building energy bills are the highest of any commercial building type. The energy demand for heating, ventilation, air conditioning, and lighting account for approximately 70% of a building’s energy use.

• They require less energy, cause less adverse environmental impacts, provide open sunlight and high quality space, improve building aesthetics, and provide new medium for architectural expression.

• Reliable, low cost technologies combined with strong marketing strategies will push solar further into the main building market. Sustainable, solar assisted low-energy solar houses are a growing part of the housing industry. Their technical performance is no longer in question so how they are marketed is critical.

• There is not only great potential to substitute fossil fuels with solar heat in buildings, but also in the industrial sector including agriculture (e.g., crop drying in developing countries).

• The potential of solar thermal technologies for the heat supply (hot water and space heat) in housing is large.

• Passive solar heating in combination with energy-efficient building construction and practices can reduce the demand for space heating up to 30%.

• Active solar can reduce the fuel demand for hot water and space heating from 50% to 70% for hot water preparation and 40% to 60% for space heating. Daylighting can reduce the electricity demand for lighting up to 50%.
• The potential for solar thermal applications in the housing sector will increase dramatically once suitable technical solutions are available to store the thermal heat for the medium to long (seasonal) term. Such advanced storage systems could utilize chemical and physical processes to reduce the total storage volume and the related costs.

Passive solar systems 

The term passive solar refers to systems that absorb, store and distribute the sun’s energy without relying on mechanical devices like pumps and fans- which require additional energy. Passive solar design reduces the energy requirements of the building by meeting either part (or all) of its daily cooling, heating and lighting needs through the use of solar energy.

Passive heating

Heating the building through the use of solar energy involves the absorption and storage of incoming solar radiation, which is then used to meet the heating requirements of the space. Incoming solar radiation is typically stored in thermal mass such as concrete, brick, rock, water or a material that changes phase according to temperature. Incoming sunlight is regulated by the use of overhangs and shades while insulating materials can help to reduce heat loss during the night or in the cold season. Vents and dampers are typically used to distribute warm or cool air from the system to the areas where it is needed. The three most common solar passive systems are direct gain, indirect gain and isolated gain. A direct gain system allows sunlight to windows into on occupied space where it is absorbed by the floor and walls. In the indirect gain system, a medium of heat storage such as wall, in one part of the building absorbs and stores heat, which is then transferred to the rest of the building by conduction, convection or radiation. In an isolated gain system, solar energy is absorbed in a separate area such as greenhouse or solarium, and distributed to the living space by ducts. The incorporation of insulation in passive systems can be effective in conserving additional energy.

Passive cooling 

Passive solar technology can also be used for cooling purposes. These systems function by either shielding buildings from direct heat gain or by transferring excess heat outside. Carefully designed elements such as overhangs, awnings and eaves shade from high angle summer sun while allowing winter sun to enter the building. Excess heat transfer can be achieved through ventilation or conduction, where heat is lost to the floor and walls. A radiant heat barrier, such as aluminium foil, installed under a roof is able to block upto 95% of radiant heat transfer through the roof. Water evaporation is also an effective method of cooling buildings, since water absorbs a large quantity of heat as it evaporates. Fountains, sprays and ponds provide substantial cooling to the surrounding areas. The use of sprinkler systems to continually wet the roof during the hot season can reduce the cooling requirements by 25%. Trees can induce cooling by transpiration, reducing the surrounding temperature by 4 to 14 degrees Farenheit.

Evaporative cooling 

Evaporation occurs whenever the vapour pressure of water is lesser than the water vapour in the surrounding atmosphere. The phase change of water from liquid to the vapour state is accompanied by the release of a large quantity of sensible heat from the air that lowers the temperature of air while its moisture content increases. The provision of shading and the supply of cool, dry air will enhance the process of evaporative cooling. Evaporative cooling techniques can be broadly classified as passive and hybrid.

Passive Direct

Passive direct systems include the use of vegetation for evapo-transpiration, as well as the use of fountains, pools and ponds where the evaporation of water results in lower temperature in the room. An important technique known as ‘Volume cooler’ is used in traditional architecture. The system is based on the use of a tower where water contained in a jar or spray is precipitated. External air introduced into the tower is cooled by evaporation and then transferred into the building. A contemporary version of this technique uses a wet cellulose pad installed at the top of a downdraft tower, which cools the incoming air.

Passive Indirect

Passive indirect evaporative cooling techniques include roof spray and roof pond systems.

Roof spray 

The exterior surface of the roof is kept wet using sprayers. The sensible heat of the roof surface is converted into latent heat of vaporisation as the water evaporates. This cools the roof surface and a temperature gradient is created between the inside and outside surfaces causing cooling of the building. A reduction in cooling load of about 25% has been observed. A threshold condition for the system is that the temperature of the roof should be greater than that of air. There are, however, a number of problems associated with this system, not least of which is the adequate availability of water. Also it might not be cost effective, as a result of high maintenance costs and also problems due to inadequate water proofing of the roof.

Roof pond 

The roof pond consists of a shaded water pond over an non-insulated concrete roof. Evaporation of water to the dry atmosphere occurs during day and nighttime. The temperature within the space falls as the ceiling acts as a radiant cooling panel for the space, without increasing indoor humidity levels. The limitation of this technique is that it is confined only to single storey structure with flat, concrete roof and also the capital cost is quite high.

Earth cooling tubes 

These are long pipes buried underground with one end connected to the house and the other end to the outside. Hot exterior air is drawn through these pipes where tit gives up some of its heat to the soil, which is at a much lower temperature at a depth of 3m to 4m below the surface. This cool air is then introduced into the house. Special problems associated with these systems are possible condensation of water within the pipes or evaporation of accumulated water and control of the system. 

Earth-sheltered buildings 

During the summer, soil temperatures at certain depths are considerably lower than ambient air temperature, thus providing an important source for dissipation of a building’s excess heat. Conduction or convection can achieve heat dissipation to the ground. Earth sheltering achieves cooling by conduction where part of the building envelope is in direct contact with the soil. Totally underground buildings offer many additional advantages including protection from noise, dust, radiation and storms, limited air infiltration and potentially safety from fires. They provide benefits under both cooling and heating conditions, however the potential for large scale application of the technology are limited; high cost and poor day-lighting conditions being frequent problems. On the other hand, building in partial contact with earth offer interesting cooling possibilities. Sod roofs can considerably reduce heat gain from the roof. Earth berming can considerably reduce solar heat gain and also increase heat loss to the surrounding soil, resulting in increase in comfort.

Active solar Systems

It involves the use of solar collectors and other renewable energy systems like biomass to support the solar passive features as they allow a greater degree of control over the internal climate and make the whole system more precise. Active solar systems use solar panels for heat collection and electrically driven pumps or fans to transport the heat or cold to the required spaces. Electronic devices are used to regulate the collection, storage and distribution of heat within the system. Hybrid systems using a balanced combination of active and passive features provide the best performance.

Active heating 

In active systems, solar collectors are used to convert sun’s energy into useful heat for hot water, space heating or industrial processes. Flat-plate collectors are typically used for this purpose. These most often use light-absorbing plates made of dark coloured material such as metal, rubber or plastic that are covered with glass. The plates transfer the heat to a fluid, usually air or water flowing below them and the fluid is used for immediate heating or stored for later use. There are two basic types of liquid based active systems- open loop and closed loop. An open loop system circulates potable water itself, through the collector. In closed loop systems, the circulating fluid is kept separate from the system used for potable water supply. This system is mainly used to prevent the freezing of water within the collector system. However, there is no need to go in for such a system in India, as freezing of water is not a possibility. Also closed loop systems are less efficient as the heat exchanger used in the system causes a loss of upto 10 degrees in the temperature of water, at the same time, one has to reckon with the extra cost of the heat exchanger as well as the circulating pumps. Compared to these, thermosiphon systems are more convenient and simple. 

Convection Heating

In Thermosiphon systems, the water circulates from the collector to the storage tank by natural convection and gravity. As long as the absorber keeps collecting heat, water keeps being heated in the collector and rises into the storage tank, placed slightly above (at least 50 cm). The cold water in the tank runs into the collector to replace the water discharged into the tank. The circulation stops when there is no incident radiation. Thermosyphon systems are simple, relatively inexpensive and require little maintenance and can be used for domestic applications. 

Solar ponds have been developed ,which harness the sun's energy that can be used for various purposes including production of electricity. 

Other devices such as solar cookers, water distillation systems, solar dryers, etc. have been developed which can be used to reduce energy requirements in domestic households and in industrial applications.

Active cooling 

Absorption cooling systems transfer a heated liquid from the solar collector to run a generator or a boiler activating the refrigeration loop which cools a storage reservoir from which cool air is drawn into the space. Rankine steam turbine can also be powered by solar energy to run a compressed air-conditioner or water cooler. 

Solar refrigeration is independent of electric supply and without any moving parts, for example, Zeolite refrigerator.

In recent years, advanced solar cooling systems coupled with changed market conditions, suggest that active solar cooling will soon enter the market in a significant way. Solar assisted air-conditioning of commercial buildings is a promising concept. The advantage of solar is that the demand for cooling coincides with the availability of high solar radiation. Continued development of high performance collectors and system components will improve the cost effectiveness of higher temperature applications. Solar assisted cooling is an extremely promising technology as peak cooling consumption coincides with peak solar radiation. Now it is necessary to support its commercialization and continued R&D. With increasing demand for higher comfort levels in offices and houses, the market for cooling has been increasing steadily over the past years. Today, solar assisted cooling is most promising for large buildings with central air-conditioning systems. However, the growing demand for airconditioned homes and small office buildings is opening new sectors for this technology. In many regions of the world, air-conditioning represents the dominant share of electricity consumption in buildings, and will only continue to grow. The current technology- electrically driven chillers, unfortunately do not offer a solution as they create high electricity peak loads even if the system has a relatively high energy efficiency standard. In particular, in Mediterranean countries sales of air-conditioning equipment are dramatically increasing, and leading to electricity shortages in some areas during peak summer conditions. The obvious link, to provide the primary energy for these cooling applications using solar thermal energy, is still under development. Over the past five years, the development of technical solutions has been initiated primarily by small and medium-scale enterprises. Very promising small capacity water chillers using sorption technology have opened a new market for use of solar thermal energy as a driving heat source for summer air conditioning. And, many new system solutions for large capacity chillers have been developed providing solar heat driven building air-conditioning.

Process heat accounts for about 40% of the primary energy supply in the OECD. The major share of the energy needed by commercial and industrial companies for production and processing and for the heating of production halls is below 250°C. This low temperature level can easily be reached using solar thermal collectors already on the market. Typical applications for solar heat plants are in the food and beverage industries, the textile and chemical industries, and for simple cleaning processes, such as car washes. The low temperatures required in these processes (30°C to 90°C) means that flat-plate collectors can be used efficiently in this temperature range. Cleaning processes are mainly applied in the food and textile industries and in the transport sector. For cleaning purposes, hot water is needed at a temperature level between 40°C and 90°C. Due to this temperature range flat-plate collectors are recommended for this application. The system design is quite similar to large-scale hot water systems for residential buildings, since they work in the same temperature range and the water is drained after usage. The increasing shortages in fresh water supplies provide a huge market for solar thermal seawater desalination. The temperature ranges at which desalination processes can be operated are below 120°C and are thus well suited for solar thermal collectors. R&D is needed to develop appropriate systems and technologies for wide spread application. Summarizing, about 30% to 40% of the process heat demand could be covered with low to medium temperature solar collector systems,

Solar thermal systems have the potential to substitute oil and gas for heating and cooling - more than one third of our energy use is for heating. This is a cost effective investment as many applications are close to market entry. The solar source for solar heating and cooling technologies is large and “unlimited”. Being not very optimistic, it is estimated that about 30% to 40% of the worldwide heat demand could be covered by solar produced heat, and in Europe about 20% of the demand for heat supply. With these assumptions, the useful in the long-term (2050) about 60 EJ to 100 EJ/year worldwide and 10 EJ to 20 EJ/year in OECD Member States.

Solar Thermal Capacity in Operation Worldwide

Installed solar thermal capacity output reached 88,845 GWh, resulting in the avoidance of 39.3 million tons of CO2 emissions. At the end of 2007, the installed solar thermal capacity worldwide equaled 146.8 GWth or 209.7 million square meters.

Distribution by Application

The use of solar thermal energy varies greatly by country. 

In China and Taiwan (80.8 GWth), Europe (15.9 GWth) and Japan (4.9 GWth), plants with flat-plate and evacuated tube collectors are mainly used to prepare hot water and to provide space heating while in North America (USA and Canada) swimming pool heating is still the dominant application with an installed capacity of 19.8 GWth of unglazed plastic collectors. It should be noted that there is a growing unglazed solar air heating market in Canada and the USA aside from pool heating. Unglazed collectors are also used for commercial and industrial building ventilation, air heating and agricultural applications. Europe has the most sophisticated market for different solar thermal applications. It includes systems for hot water preparation, plants for space heating of single-and multi-family houses and hotels, large-scale plants for district heating as well as a growing number of systems for air conditioning, cooling and industrial applications.

In Austria, Germany, Switzerland and the Netherlands the share of applications other than hot water preparation in single-family houses is 20% and higher than in other European countries. There are about 130 large-scale plants (500m2; 350 kWth) in operation in Europe with a total installed capacity of 140 MWth. The biggest plants for solar assisted district heating are located in Denmark with 13 MWth (18,300 m2) and Sweden with 7 MWth (10,000 m2). The biggest reported solar thermal system for providing industrial process heat was installed in 2007 in China. This 9 MWth (13,000 m2) plant produces heat for a textile company.

Leading Countries

Flat-plate and evacuated tube collectors

Based on the total capacity of flat-plate and evacuated tube collectors in operation at the end of the year 2007, the leading countries are: 

• China (79.9 GWth)
• Turkey (7.1 GWth)
• Germany (6.1 GWth)
• Japan (4.9 GWth)
• Israel (3.5 GWth)
• Brazil (2.51 GWth)
• Greece (2.50 GWth)
• Austria (2.1 GWth)
• USA (1.7 GWth)
• India (1.5 GWth)

As can be seen China is by far the largest market, representing 66% of the world market of flat-plate and evacuated tube collectors. Here it should also be mentioned that China again increased its market share by 2% in 2007. Based on the market penetration – total capacity in operation per 1,000 inhabitants – the leading countries are 

• Cyprus (651 kWth)
• Israel (499 kWth)
• Austria (252 kWth)
• Greece (224 kWth) 
• Barbados (197 kWth) 
• Jordan (100 kWth)
• Turkey (95 kWth)
• Germany (73 kWth)
• China (60 kWth) and 
• Australia (57 kWth).

Unglazed plastic collectors

For the heating of swimming pools using unglazed plastic collectors, the USA leads with a total capacity of 19.3 GWth in operation ahead of Australia with 2.8 GWth, Germany and Canada with 0.5 GWth each, and Austria and South Africa with 0.4 GWth. The market penetration – total capacity in operation per 1,000 inhabitants – gives a slightly different picture. The lead countries are: 

• Australia 137 kWth 
• USA 63 kWth 
• Austria 51 kWth 
• Switzerland, Netherlands and Canada with an installed capacity between 20 and 14 kWth per 1,000 inhabitants.

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