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Solar thermal systems are converting solar irradiation into heat with high efficiency of up to 80%. However, what counts is the amount of fuel which is saved by the solar thermal system. One would expect that it is simple to evaluate the fuel savings, however it is more complicated then it looks like.
India is a sunny country, however the solar irradiation is not everywhere the same. In most parts of India, the annual average global solar irradiation on a horizontal area with 1 square meter is between 4.5 and 6.0 kWh per day this means 1,700 and 2,200 kWh per year.
The solar irradiance is a dimension of the energy delivered by the sun during a specific time on a specific area. In comparison, the solar irradiance is the power of the sun at a specific moment on a specific area. On a sunny day with a clear sky the solar irradiance is everywhere about 1000 W/m2 (Watt per square meter) on an area perpendicular to the radiation. The variation of the solar irradiation is mainly caused by the angle with which the irradiance falls on a surface, this is why the irradiation is lower at higher latitudes, and by the weather conditions, this is why in desert areas like Rajasthan with dry air and clear sky the irradiation is higher and in cloudy areas along the west coast it is lower.
The solar irradiation is composed of Direct Irradiance and Diffuse Irradiance and is called Global Irradiance. Summed up over a specific time it becomes the Global Irradiation. Concentrated solar thermal collectors can only use Direct Irradiation, therefore maps with Direct Normal Irradiation (DNI) are used. However, SWHS can use Diffuse Irradiation as well, therefore the Global Horizontal Irradiation (GHI) map is used.
Since a solar collector is oriented towards the sun, it receives a higher global solar irradiation than the horizontal surface. The solar irradiation received by the collector surface can be calculated from the horizontal irradiation by taking into account the angle and the orientation of the solar collector and the latitude where it is installed.
The solar irradiance is passing the cover glass of the solar thermal collector and is converted by the absorber into heat. Typically 20% to 30% of the energy is lost by reflections on the glass and the absorber. These losses are called optical losses.
Of the heat generated, one part gets lost through the collector housing, the other part, the useful solar heat, is transported by the heat transfer fluid (water or water-glycol mix) to the storage or to the point of use. The thermal losses of the solar collector are increasing with the temperature difference between the absorber and the ambient temperature. As higher the temperature of the absorber is, as higher are the losses. But as better the collector is insulated, as lower are these losses.
The temperature of the absorber is a key value for the collector efficiency. But it does not only depend on the solar irradiation and the quality of the collector, but also on the inlet temperature of the heat transfer fluid. The absorber is increasing the temperature of the fluid, but if the water flowing into the collector is cold, the resulting absorbertemperature is lower than if the inlet temperature is high, e.g. if it comes from a storage, which is already hot. If the temperature is lower, the same energy can be transferred with a higher efficiency.
The efficiency of a flat plate collector is at maximum about 75%, at an ambient temperature of 30°C and an operation temperature of 60°C it is about 50% at an irradiation intensity of 800 W/m2.
The heat transfer fluid (which can be water or freeze protected water-glycol mixture in cold regions) is circulating in pipes to the heat storage and the point of use. Some of the energy gets lost during the transport depending on the thermal insulation of the pipes.
A tank is typically installed to store the solar hot water since there is usually a mismatch between the solar water heating and the hot water usage. The hot water storage tank is loosing heat depending on the thermal insulation, the temperature of the hot water and the duration how long the heat is stored.
The ability of the solar thermal system to deliver thermal energy depends also on the demand side. If the heat demand is low in relation to the ability of the collector area to generate heat, then the temperature in the storage and therefore the collector inlet temperature and the absorber temperature are high. This leads to high thermal losses of the collector.
In addition, the temperature needed by the user plays a role. If only high temperatures can be used, the collector field is only working at higher radiation and at higher temperatures and therefore lower efficiency. If all temperature levels can be used, as it is the case with preheating of feeding water for steam boilers, the efficiency is higher.
The description shows, that a lot of factors influence the overall efficiency of the solar thermal system. A good system efficiency can only be achieved, if all components are working properly and the SWHS is designed well. The system efficiency of a SWHS is typically between 20% and 40%. This means that 20% to 40% of the energy of the solar irradiation falling on the collector surface during one year is delivered to the point of use and replacing fossil fuels.