SOLAR THERMAL ENERGY
Although most research into the use of solar energy in recent years has been on photovoltaic technology, where sunlight is converted directly into electricity, there are many applications of solar thermal energy such as heating, drying and water distillation. Many solar thermal technologies have existed for centuries and are well understood. They have established manufacturing bases in many sun-rich countries. Unlike photovoltaic technologies manufacturing can be done on a small scale without using expensive equipment. More sophisticated solar thermal technologies do exist that generate electricity (often on a large scale) but these are not covered in this technical brief. Solar technologies that rely entirely on energy absorb from the sun and have no moving components, are referred to as passive solar technologies where as active solar technologies may have some additional input such as a pump to drive the system. The nature and availability of solar ra diation Solar irradiation, or insolation is the “rate of delivery of direct solar radiation per unit of horizontal surface”, measured in W/m2. (Merriam-webster.com) The earth revolves around the sun with its axis tilted at an angle of 23.5 degrees. It is this tilt that gives rise to the seasons. The strength of sun is dependent upon the angle at which it strikes the earth’s surface, and so, as this angle changes during the year, so the solar insolation changes. Thus, in northern countries, in the depths of winter, where the sun is low in the sky to the south, the radiation strikes the earth’s surface obliquely and solar energy is low. The two phenomena described above provide an explanation for the variations of solar irradiation with season and lattitude.
23.5° max Winter
Figure 1: The angle of the earth to the sun changes throughout the year.
Illustration: Practical Action / Neil Noble
The total solar irradiation received in a day can vary from 0.5 kWh/m2/day in the UK winter to 5 kWh/m2 in the UK summer and can be as high as 7 kWh/m2/day in desert regions of the world, such as regions of Nigeria (Solar Water Heating in Nigeria, 2006) and the Sahara in Algeria. (Survey of Energy Resources, 2010) Many tropical regions do not have large seasonal variations and receive an average 6 kWh/m2/day throughout the year. The diagram below shows the approximate percentages of direct and diffuse solar insolation that reaches the surface of the earth. As the direct insolation forms a larger proportion of the total received, it follows that varying factors such as the weather, i.e. cloud cover, and the time of day will greatly affect the amount of solar insolation reaching the surface of the earth (Powerfromthesun.net). It is interesting to note that whilst both direct and diffuse radiation is useful, diffuse radiation cannot be concentrated.
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Solar thermal energy
Figure 2: Dispersion of solar irradiance through the atmosphere. Action (powerfromthesun.net).
Illustration: Neil Noble / Practical
Although potentially obvious, it is also useful to note that the majority of the solar water heating will occur during daylight hours. This is not necessarily when it is demanded, thus solar hot water storage tanks are normally required. Daily, seasonal and geographical variations in solar insolation are an important aspect of solar energy because of the influence on system design and solar energy economics. A useful document which summarises the extent of the application of solar energy in 43 countries around the world is the 2010 Survey of Energy Resources by the World Energy Council. The document also notes the levels of solar radiation that can generally be expected in the countries listed which may be useful as a quick reference. Alongside journals and books, another useful source detailing solar insolation levels around the world is a tool which has been developed by NASA and is free for public use. Below are images produced by this tool which show the global variation in solar insolation.
Solar thermal energy
Figure 3: Examples of average monthly global variation of solar insolation in a year (NASA Earth Observations Website) The analysis tool enables users to quantitatively investigate the variations in solar insolation in a particular region throughout the year. Three snapshots of either a daily average, average across eight days or monthly average, can be compared at once and presented in several different ways; either a probe, which gives the insolation at a particular point on the globe, a transect through a region (see Figure 4) or an average across a defined region. All three methods can be specified by the user.
Comparison of the variation in average solar insolation with distance across the transect The The average solar insolation during 3no months is compared
Transect specified for investigation
Interactive interface of analysis tool
Figure 4: Screenshot of NASA Earth Observations Solar Insolation Analysis Tool Each set of data can also be translated into the Google Earth software. This can be used to gain an overall impression of global solar insolation variation at a particular time.
Figure 5: Data from NASA Earth Observations viewed in Google Earth
Solar thermal energy
Solar thermal energy applications
Solar energy reaches the earth’s surface as short wave radiation, absorbed by the earth and objects on the earth that heat up and re-radiated as long-wave radiation. Obtaining useful power from solar energy is based on the principle of capturing the short wave radiation and preventing it from radiating away into the atmosphere. For storage of this trapped heat, a liquid or solid with a high thermal mass is used. In a water heating system this will be the fluid that runs through the collector, whereas in a building the walls will act as the thermal mass. Pools or lakes are sometimes used for seasonal storage of heat. Glass will allow short wave radiation to pass through it but prevents long wave radiation heat escaping. If this energy is being used to heat water with a collector panel, then the tilt and orientation of the panel is critical to the level of energy captured and hence the temperature of the water. The collector surface should be orientated towards the sun as much as is possible. Most solar water-heating collectors are fixed permanently to roofs of buildings and cannot be adjusted. More sophisticated systems for power generation use tracking devices to follow the sun through the sky during the day. There are many methods available for aiding system design and for predicting the performance of a system. The variability of the solar resource is such that any accurate prediction requires complex analytical techniques. Simpler techniques are available for an approximate analysis. Water heating The most common use for solar thermal technology is for domestic water heating. Hundreds of thousands of domestic hot water systems are in use throughout the world, especially in areas such as the Mediterranean and Australia where there is high solar insolation (the total energy per unit area received from the sun). Presently, domestic water heaters are usually only found amongst wealthier sections of the community in developing countries.
Low temperature (below 100ºC) water heating is required in most countries of the world for both domestic and commercial use. There are a wide variety of solar water heaters available. The simplest is a piece of black plastic pipe, filled with water, and laid in the sun for the water to heat up. Simple solar water heaters usually comprise a series of pipes that are painted black, sitting inside an insulated box fronted with a glass panel, this is known as a solar collector. The fluid to be heated passes through the collector and into a tank for storage. The fluid can be cycled through the tank several times to raise the heat of the fluid to the required temperature. There are two common simple configurations for such a system and they are outlined below. The thermosyphon system makes use of the natural tendency of hot water to rise above cold water. The tank in such a system is always placed above the top of the collector and as water is heated in the collector it rises and is replaced by cold water from the bottom of the tank. This cycle will continue until the temperature of the water in the tank is equal to that of the panel. Where there is a main water supply fresh cold water is fed into the system from the mains as hot water is drawn off for use. A one-way valve is usually fitted in the system to prevent the reverse occurring at night when the temperature drops.
F igure 1 Solar water heaters in Nepal. Photo: Practical Figure 6: 6: Solar water heating in Nepal. Photo: Action. Practical Action
Solar thermal energy
Open loop systems allow water to run through the solar panels and be stored in the storage tank to be used. Closed loop systems are where the water that circulates through the solar panel is separate from the water used. The system uses a heat exchanger. This means that anti freeze can be added to the water running through the panels which allows them to be used in cold climates. Atmospheric systems are used where there is no mains water delivery to the storage tank so as water is taken from the hot water tank it is replaced from an additional cold water tank that is located above it. A break pressure valves allows water to feed the hot water tank when required. Atmospheric systems can be open loop or closed loop. Batch solar water heating systems are used as a simple approach to obtaining hot water. The system is filled with water and left to heat up. Once the water is heated up it can be used as required but system has to be refilled manually. Simple Solar Water Heater for Developing Countries A. Jagadeesh, Homepower Issue 76 http://www.homepower.com/ More than 90% of systems worldwide are based on the thermosyphon principle. Pumped solar water heaters use a pumping device to drive the water through the collector. The advantage of this system is that the storage tank can be sited below the collector. The disadvantage of course is that electricity is required to drive the pump. Often the fluid circulating in the collector will be treated with an anti-corrosive and /or anti-freeze chemical. In this case, a heat exchanger is required to transfer the heat to the consumers hot water supply.
Integrated systems combine the function of tank and collector to reduce cost and size. Water heating systems can be made relatively simply while more sophisticated systems are available at a higher price. Evacuated tube collectors have the heat absorbing element placed within an evacuated glass sheath to minimise heat losses. System complexity also varies depending on use. For commercial applications, banks of collectors are used to provide larger quantities of hot water as required. Many such systems are in use at hospitals in developing countries. A solar pond is an approach that uses large bodies of water to collect and store solar thermal energy with relatively little equipment. The pond uses the principle that slat water is heavier than fresh water so a layer of salt water at the bottom of the pond traps the heat energy and the temperature can rise above 90°C. Solar ponds can be used to provide heating to houses but can also be used for other applications. For example, using a low temperature turbine the solar pond can be used to generate electricity or it can be used to provide power to a water distillation unit as developed by The University of Texas. Bhuj, Gujarat has the largest operating solar pond in India covering an area of 6000 m2 which is used to supply process heat to the Kutch dairy. The solar pond was developed by the Gujarat Energy Development Agency (GEDA), the Tata Energy Research Institute (TERI) and the Gujarat Dairy Development Corporation (GDDC). http://www.teriin.org/tech_solarponds.php
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