Solar thermal technology can be used to provide heating for domestic or industrial uses. A solar heating system can capture the sun's radiation and use it for both hot water heating and supplement home heating by piping hot water through traditional or modern radiators, furnaces, or use it in hydronic system for in floor radiant heat. In most cases solar should be used with conventional power as a supplement, this way you never have to rely completely on the sun shinning. An important part of a home solar system is the controller. The controller senses input and output temperatures. By using a digital control center and diverter valves, hot water can be controlled as a supplementary energy source when the solar power is available.
A solar heating system can capture the sun's radiation and use it for both hot water heating and supplement home heating by piping hot water through traditional or modern radiators, furnaces, or use it in hydronic system for in floor radiant heat. In most cases solar should be used with conventional power as a supplement, this way you never have to rely completely on the sun shinning. An important part of a home solar system is the controller. The controller senses input and output temperatures. By using a digital control center and diverter valves, hot water can be controlled as a supplementary energy source when the solar power is available.
Solar Home Heating uses the solar collectors to capture the sun's energy, this energy is then transfered to a storage tank or concrete pad (in floor heating). The stored heat is then integrated with the existing heat system to supplement the heat supply. A larger storage tank can hold more energy and as such can be used in the evening to supply heat when the sun is not shinning. Because the heat demand occurs during the winter, a secondary dissipation loop may be needed to off load the heat in the summer months. Alternatively, this energy can be off loaded to heat other heat demand sources such as pools, hot tubs or domestic water heat. A dissipation circut will also help regulate the storage tank temperature allowing the home heating circuit to be set at a comfortable level without over-heating (see http://www.solartubs.com/solar-home-heating/ ).
A solar space-heating system can consist of a passive system, an active system, or a combination of both. Passive systems are typically less costly and less complex than active systems. However, when retrofitting a building, active systems might be the only option for obtaining solar energy. Passive solar space heating takes advantage of warmth from the sun through design features, such as large south-facing windows, and materials in the floors or walls that absorb warmth during the day and release that warmth at night when it is needed most. A sunspace or greenhouse is a good example of a passive system for solar space heating.
Active solar space-heating systems consist of collectors that collect and absorb solar radiation combined with electric fans or pumps to transfer and distribute that solar heat. Active systems also generally have an energy-storage system to provide heat when the sun is not shining. The two basic types of active solar space-heating systems use either liquid or air as the heat-transfer medium in their solar energy collectors.
An overview of examples of SWH projects applied in industrialised countries can be found on the IEA Solar Heating and Cooling  Internet site. Even though solar thermal is used today mainly for providing hot water to households and pools, Vannoni et al (2008) demonstrate its significance in the final energy consumption in the industrial sectors. A great share of the industries' heat demand is needed in the low and medium temperature range, such as food – including wine and beverage, textile, transport equipment, metal and plastic treatment, chemical and for several processes (cleaning, drying, evaporation and distillation, blanching, pasteurisation, sterilisation, cooking, melting, painting, surface treatment).
Most heating system require some form of freeze protection in the winter months, thus the solar loop usually contains a mixture of water and glycol to prevent freezing. Specially solar storage containers have been designed for buffering the solar heat with the home heat. These storage tanks have an internal heat exchangers built into them. The solar loop is found in the bottom of the tank and heats the water. Heat is extracted the heat from the top of the storage tank to the heat load area such as radiant floor heat by activating a second pump. When there is demand and the top of the tank is hot enough to meet this demand the pump will turn on. By using a solar storage tank, the system can retain the heat that would otherwise be dissipated once the demand temperature has been met.
While industrial applications of the technology are low at the moment, the potential is great as the industrial demand for heating is mainly below 250ºC. In particular, the low temperature range in industrial heating demand (less than 80ºC) is well within the range already available for commercial solar thermal collectors (Weiss and Rommel, 2005). The heat can, for instance, be used in industrial processes such as crop drying and in desalination plants.
For domestic use, several forms solar thermal systems can be applied. A SWH system can be installed in the roof of a house with solar collectors that can either be flat plate, or evacuated tubes (efficiencies of 30% and 40%, respectively) and that are coupled to a storage tank for the hot water. Where there is a possibility of low outside temperatures then either a drain back or closed loop system with an intermediary fluid is used with a heat exchanger system (Indirect method). Where there is no freezing risk, the water is circulated and heated directly (Direct method).
As with all technologies there is a need for training and quality control programmes as well as information provision and preferably some form of policy to promote and require the technology installation. Suppliers also need to be fostered along the supply chain, though the basic hardware for SWH is simple and commonly available. The solar collector, however, needs a specialist manufacturer.
The main barriers to the implementation of the technology identified by ESTIF (2003) are as follows:
- Higher up front costs than conventional technologies, although ongoing costs are lower,
- It is not yet perceived as a standard option as solar thermal is not yet integrated into mainstream heating and construction sectors,
- Pay-back times may be long depending on configuration and location,
- Higher transaction costs for information procurement and installation,
- Awareness of Solar thermal options and their benefits is low,
- A skilled installer base needs to be fostered,
- Insufficient harmonisation of standards; the EU Solar keymark will improve this situation,
The markets in China, Australia, New Zealand and Europe are the fastest growing with a 25% growth rate in China and Taiwan followed by 19% in Australia and New Zealand then 13% in Europe. In Japan an estimated 10,000,000 homes use this technology to heat their homes. In North America solar water heating is quickly growing as the price of solar collectors fall and governments support the use of clean energy, financially. Swimming pool applications also recorded an increase.
With a view to the application of the technology in Russia, where presently no market development is taking place, a meeting was organised by EREC and the Russian Energy Technology Centre in May 2007 on ‘Perspectives for solar thermal energy in Southern Russia ’.
There have been many developments and variation in the configuration and materials used in the SWH technology, such as:
- use of a roof as collector,
- new materials as storage,
- new building designs as storage,
- large scale for district heating, and
- new applications such as desalination.
The potential of SWH in the EU to replace conventional energy sources is large; the technical potential in the EU is 300 million m2 of collectors (equivalent to 210 GWth). If 30% of this potential was achieved, then it has been estimated that this would reduce EU CO2 emissions by 6% of the Kyoto commitment. Thus, at present the EU uptake is in the region of only 5% of the possible uptake.
In the EU, especially within the framework of IEA, there is continuous assessment and development of the technology. According to Weiss et al. (2006), in 2004, Europe had 10.8 GWth (equivalent to 15.4 million m2 mainly flat plate and evacuated tube collectors, where the conversion factor for collector area to solar thermal capacity is 0.7 kWth/m2). The technical potential in the EU is 300 million m2 of collectors (equivalent to 210 GWth). It has been estimated that if 30% of this potential was achieved, then this would reduce EU CO2 emissions by 6% of its Kyoto Protocol commitment (reducing CO2-eq emissions by 8% below 1990 levels). Thus, at present the EU uptake is in the region of only 5% of the possible uptake.
Several examples of such installed technologies exist worldwide. For instance in India, Tube Investments (TI ) Cycles has installed a solar air heating system at a bicycle factory in Chennai (see http://www.solarthermalworld.org/node/1132 ), where in reference to the first year's operational results, the estimated Indian Rupees (INR) 2 million investment will be repaid in less than two years, supported by a national capital subsidy of 1,750 INR/m2 of collector area. Moreover, the company profited from a 80 % accelerated depreciation on investment costs in this first year.
The social acceptability of the technology is high with no problems reported as it is a benign technology on the roof of buildings. According to ESTIF (2006), if the required political and market conditions were put in place, 580 000 full time jobs would be created by 2030 related to producing, operating and maintaining SWH systems. These would be local jobs in manufacturing, engineering, installation and maintenance. Relative to coal-based and nuclear energy, the number of jobs per 1000 GWh of supplied energy is 90 for hard coal ,72 for nuclear energy, and 3960 for solar thermal.
In terms of economic and social benefits, the technology can contribute to sustainability especially as it is relatively simple and could be delivered by locally trained installers with local supply chains in the main. This would lead to jobs and to a better quality of life, as well as minimise energy bills and dependency on fossil fuel imports with their corresponding vulnerability to price fluctuations. It is a safe technology avoiding fire and other health risks. It is particularly applicable to developing countries for space cooling and hot water.
China is the major developing country using solar thermal, as discussed above, and represents one-third of the installed capacity worldwide. Brazil is next with 1.59 GWth capacity. Other countries, such as Barbados, Mexico and South Africa have much lower capacities installed. Although the potential for use of the technology in developing countries is very high, only few countries have any existing installed capacity. This is an obvious area for expansion for use to supply needs and minimise import of fuels.
Low-temperature water heating is an important contributor to lowering energy demand for heat in the residential, public and industrial sectors. 41 countries currently use solar thermal technology with an estimated installed capacity in 2005 of 115 GWth and an annual collector yield of 58,117GWh (209,220 TJ) (Weiss et al, 2006). In the USA and Canada the technology is mainly used for swimming pool heating using unglazed plastic collectors (in the USA the solar heating capacity for this service amounts to 18.4 GWth) while the other countries use flat plate and evacuated tube collectors for hot water and space heating. The leading countries for solar-based water and space heating are China (43.4 GWth), Japan (5.4 GWth), Turkey (5.1 GWth), Germany (4.53 GWth) and Greece (2.1 GWth) in terms of capacity. Also in other countries it is considered a major end use for energy, e.g., in Brazil the residential sector alone accounts for 24TWh/y installed capacity.
The use of solar thermal as indicated above could have a major impact on the need for imported supplies of fuel so that security of energy supply would be improved. Most of these technologies lead to savings though some developments still need to be made commercially competitive. Maintenance costs are very low. In addition, many countries have a programme of subsidies to develop demand and are improving training and quality assurance of suppliers. Solar thermal is not a complex technology for space heating and cooling and hot water provision and it has been available for many years, though it is improving all the time. Systems have been shown to work for decades.
In terms of other benefits for the environment the technology avoids the use of fossil fuels and their emissions, while being a silent technology.
According to Weiss et al. (2006), the contribution of solar thermal systems to energy supply of 58,117 GWh mentioned above corresponds to a total annual avoidance of 25.4 Mt CO2. The use of the technology does not produce GHGs, although life cycle implications (construction of different parts) are unknown.
For calculation of these GHG emission reductions, it is recommended to apply the approved methodology for thermal energy production with or without electricity  project (small scale activities) which has been developed under the Clean Development Mechanism of the UNFCCC Kyoto Protocol (CDM). This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored. General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html .
According to Philibert (2005), costs of SWH systems can vary greatly from country to country. In Greece a domestic solar thermal hot water system for a one-family unit (2.4 m2 collector area and 150 litre tank) costs € 700, but in Germany with a 4-6 m2 area and 300 liter tank it costs € 4,500. Conversely, a system for space heating will provide greater savings in the north than the south. In northern France, savings of €730-900 per year are quoted compared to € 120-180 in the southern part of the country.
The EU calls for 100 million m2 of solar thermal surface by 2010 helps to stimulate interest and the market but subnational programmes are available. For example, the German Solarthermie 2000 plus aims to increase the annual solar contribution to hot water and heat production from individual installations from 10-30% to 60%. Seasonal storage is targeted with grants of up to 50%.
The new certification and quality standard the Solar Keymark  is now being introduced to harmonise standards across the EU which will encourage growth through reliability.
The IEA as been a prime mover in this area since 1977. It has a Solar Heating and Cooling  programme responsible for a series of R&D and policy tasks for the implementation of the technology, such as the solar resource knowledge management task which complements the UNEP Solar Wind Energy Resource Assessment (SWERA). Another area is solar assisted cooling systems to improve conditions for market introduction for small and large-scale applications, as demonstrated by the IEA .
EU Intelligent Energy programme  under the sub-programmes of SAVE, ALTENER and COOPENER have funded projects on solar thermal. The EU Altener project Soltherm , which aimed at achieving a situation with a solar water heater for every European, has ended, but has initiated a European network of experienced and new market players and has created new marketing campaigns to boost the solar thermal market. It has also provided a forum for exchanging experience and improved ongoing activities in the market through insights gained.
At the end of May 2006, the European Solar Thermal Technology Platform (ESTTP ) was launched by the European Solar Thermal Industry Federation  and the European Solar Energy Research Centres Agency (EUREC ). It aims to develop a comprehensive strategy for research and market deployment of solar thermal technologies in Europe by providing a platform for the exchange of expertise and knowledge.
Worldwide, there are many programmes run by organisations such as the USH2O, REEEP, UNFCCC/TTCLEAR and ADB Finesse project programmes. Two recent studies funded by REEEP are directly relevant here and address one of the main issues for new and clean energy sources; the high up front costs of some systems.
Besides financing models such as those described, other ways to move the technology forward are:
- Renewable Energy Certificate  trading is also seen as a viable way forward and a way of bringing these technologies into the green power market.
- The Renewable Energy Financing in the Mediterranean Region Project (MedREP ) initiated in 2002 by UNEP with the support of the Italian Ministry of Environment and Territory is designed to increase investment in the renewable energy sector in the southern Mediterranean initially in Tunisia, Morocco, and Egypt. The MedREP study will investigate options for increasing finance flows to renewable energy companies and projects in target countries as well as develop support mechanisms for lenders and investors to finance larger scale commercialisation. Possible mechanisms include seed or patient capital funds, interest rate subsidies, and investment advisory support facilities.
- The Rural Energy Enterprise Development (REED ) initiative operated by UNEP has several programmes. AREED (African REED) is one example with the aim of developing new sustainable energy enterprises. It offers rural energy enterprises in Mali, Ghana, Tanzania, Senegal and Zambia enterprise development services and start-up funding.
- UNEP DTIE  offers finance for interest rate subsidies and investment advisory support facilities.
- BASE  is a not for profit organisation which in collaboration with UNEP ‘helps to build strategic partnerships between entrepreneurs and financiers to mobilize capital for sustainable energy in both developing and industrialized countries’.
[this information is kindly provided by the UNEP Risoe Centre Carbon Markets Group ]
Project developers of solar thermal energy projects in the CDM pipeline mainly apply the following methodologies:
As of 1 February 2010, there are 10 solar thermal projects in the CDM pipeline - 5 are registered and 5 are at the validation stage.
European Solar Thermal Industry (ESTIF), 2003. Sun in Action II - a Solar Thermal Strategy for Europe.
European Solar Thermal Industry (ESTIF), 2006. Solar Thermal Markets in Europe.
European Solar Thermal Industry (ESTIF), 2009. Solar Thermal Markets in Europe – Trends and Market Statistics 2008.
Mangold, D. and Schmidt, T., 2006. New steps in seasonal thermal energy storage in Germany.
Philibert, C., 2005. The present and future use of solar thermal energy as a primary source of energy, IEA, Paris, France.
Vannoni, C., Batisti, R. and Drigo, S., 2008. Potential for solar heat in industrial processes. IEA SHC Task 33 and SolarSpaces Task XIV report. University of Rome, Italy.
Weiss, W. and Rommel, M., 2005. Medium temperature collectors: State of the art, within Task 33/IV for IEA SHC programme, May.
Weiss, W., Bergmann, I. and Faninger, G., 2006. Solar heat Worldwide - Markets and contribution to the Energy Supply 2004. Report for International Energy Agency Solar Heating and Cooling programme.