Overview of Low Temperature Solar Thermal Energy Conversion Applications Prepared by Prof. Dr. A. R. El-Ghalban Department of Mechanical Engineering University of Engineering and Technology Taxila, Pakistan
Low Temperature Solar Thermal Energy Conversion Applications • Low temperature solar thermal systems collect solar radiation to heat air and water for industrial applications including: • Water heating. • Domestic Hot Water. • Industrial and Process Heat. • Swimming Pool Heating.
Low Temperature Solar Thermal Energy Conversion Applications • Space heating, ventilation and cooling. • Solar cooking. • Water desalination. • Crop drying. • Power generation.
Water heating • Solar water heater systems are a well-tried and tested technology. • They are suitable for both new-build and retrofit. • A system will typically provide 60-70% of domestic hot water needs over a year. • There are many possible designs for a solar water heater.
Components of Solar Water Heaters • In general, it consists of three main components: • Solar collector, which converts solar radiation into useable heat. • Heat exchanger, pump, controller module, which transfers the heat from the solar collector into the potable water. • Storage tank to store the solar heated water.
Solar Collectors • There are two types of solar collectors: • Flat plate solar collector. • Unglazed flat plate solar collector. • Glazed flat plate solar collector. • Evacuated tube solar collector.
Types of water circulating • Water circulating might be passive and active systems. • Passive systems rely on gravity and the tendency for water to naturally circulate as it is heated, allowing water or heat-transfer fluid to move through the system without pumps. • Because they contain no electric components, passive systems are generally more reliable, easier to maintain, and possibly longer-lasting than active systems.
Advantages of passive solar water heaters: • Passive circuits are that they don’t rely on electrically powered pumps to circulate the heat-transfer fluid and they are relatively cheap. • Disadvantages of passive solar water heaters : • They require careful planning to optimize performance, they are prone to sluggish performance and there is a poor control of over-heating. • The hot water storage tank needs locating above the collector level
Active circulation ‘Pumped circulation’ • The pump circulates the heat-transfer fluid from the collector panels through the heat exchanger in the hot water cylinder and back to the solar collectors for re-heating. • The temperature sensors ensure that fluid is only circulated when the fluid in the collectors is hotter than in the cylinder.
Advantages of active circulation: • Integral protection against freezing • Overheat control • Heat is delivered from the collector at optimal rate • Greater choice of collector and pipe layout • Reduces heat loss through pipes
Disadvantages of active circulation: • Increased complexity • Pump requires electricity (though this can be alleviated by PV supply) • More expensive
Active solar heating circuits • Primary circuits transfer may be Direct (Open) or, the more usual Indirect (closed): Direct (Open) circuits • Direct circuits are those that directly heat the water that flows from the household taps. They are rarely used. • Advantages of direct circuits : • Simplicity and increased efficiency over indirect circuits. through reduction of heat transfer loss.
Disadvantages of direct circuits : • They are subject to freezing unless the water is drained-back when the pump switches off, which puts constraints on the positioning of the collectors in relation to the feed tank. • As new water continually flows through the collectors, they can be prone to ‘furring’ in the collector waterways resulting in loss of efficiency. • Pump requires electricity (though this can be alleviated by PV supply) • More expensive
Indirect (closed) circuits • Most circulation systems are indirect. Indirect circuits use a separate ‘heat-transfer fluid’ circuit to transfer heat from the collectors to the pre-heat cylinder. Their main advantage is that they can employ a wide range of materials and fluids as part of the circulation. There are different types of circulation that can be used:
Space heating, ventilation and cooling. • Active space heating. • Passive space heating and cooling. • Passive space ventilation. • Space air conditioning. • Solar cooking. • Water desalination. • Crop drying. • Power generation.
Active solar space heating • Water Space Heating • Air Space Heating
Passive Solar Space Heating • passive solar heating allows the sun to do all the work. • That is, there is no additional mechanical assistance. • In cold climates, south-facing windows designed to let the sun's heat in while insulating against the cold are ideal. • In hot and moderate climates, the strategy is to admit light while rejecting heat.
Passive Solar Space Heating • Passive solar heating system may be direct or indirect gain. • Direct solar gain system • The sun's heat is stored by the building's inherent thermal mass in materials such as concrete, stone floor slabs, or masonry partitions that hold and slowly release heat.
Passive Solar Space Heating • Direct solar gain system • In this direct gain design - A direct gain design with an interior water wall for heat storage. Heat stored in the water wall is radiated into the living space at night. • Diffusing glazing materials. Translucent glazing scatters sunlight to all storage surfaces
Passive Solar Space Heating Indirect Solar Gain System • Indirect gain water wall collects and stores heat during the day. Heat stored in indirect gain water wall is radiated into the living space at night. • Indirect gain Trombe wall stores heat during the day. Excess heat is vented to the interior space. At night Trombe wall vents are closed and the storage wall radiates heat into the interior space.
Passive Solar Space Heating Indirect isolated Solar Gain System • Attached greenhouse with vented storage wall. Heat is stored in the wall during the day - excess heat is vented to the interior space. At night the wall vents are closed and stored heat is radiated to both the greenhouse and the interior space.
Passive Solar Space Heating Indirect isolated Solar Gain System • Heating cycle - Roof pond collects and stores heat during the day. At night roof ponds are covered and stored heat is radiated into the space below.
Passive Solar Space Cooling • An indirect gain mass wall can be used to significantly increase ventilation rates in adjoining spaces. • An overhang above a south window will shade the window completely from early May to mid-August, yet allow for winter sun access. Shading devices should be sized using the given graphic method.
Passive Solar Space Cooling • Roof bonds utilizing cool, clear night-skies can provide total cooling. Panels are kept closed during the day and opened after dusk to radiate out the absorbed day time interior heat. • An overhang above a south window will shade the window completely from early May to mid-August, yet allow for winter sun access. Shading devices should be sized using the given graphic method
Passive Solar Space Cooling • Open pond with water wall - combined systems can be devised to provide direct cooling for all interior spaces.
Passive Solar Ventilation • Thermal chimneys can be constructed in a narrow configuration (like a chimney) with an easily heated black metal absorber on the inside behind a glazed front that can reach high temperatures and be insulated from the house.
Solar Cooking • Solar cooking and baking are easy passive solar energy application. Solar cookers are safe around children and provide a great way to learn about and use solar energy. Solar cookers are clean, convenient, non-polluting and easy on the environment.
Low cost Low temperature Rugged Lightweight Seasonal pool heating Unglazed Flat Plate Solar Collectors
Moderate cost Higher temperature operation Can operate at mains water pressure Heavier and more fragile Glazed Flat Plate Solar Collectors
Higher cost No convection losses High temperature Cold climates Fragile Snow is less of a problem Installation can be more complicated Evacuated Tube Collectors
Passive solar water heating system • Easy to install and maintain; no moving parts • Storage tank must be installed above or close to collector • Uses no electricity;
Active solar heating system • The main components on an active solar water heating system are • Solar collector • A circulating system • Storage tank • Back up heating system • Control system
Active space heating system The system components in an active space heating application are the same for water heating with the addition of radiators for space heating or under floor heating coils or even forced air systems.
Active air space heating system A transpired air collector preheats air for building ventilation by using a fan to draw fresh air through the system. Outside (ambient) air passes through holes in the collector (absorber) and is heated as it is drawn up the air space (plenum) between the collector and the south wall of the building.