Solar Heating/Cooling/Dehumidifier Systems TECH 581 – Solar Energy Systems Summer 2009 Module 2-5 – Solar Thermal Applied Solar Heating/Cooling/Dehumidifier Systems Energy for heating buildings and hot water consumes approx. 1/4th of the annual energy production in USA. Please read Sec. 5.1 for the calculation of heating and hot water loads in buildings. Solar Water Heating Systems Basically two types: a) Thermo-syphon - passive systems (natural circulation): in non-freezing climates b) Active systems (forced circulation): in freezing climates and commercial and industrial process heat.

Thermo-syphon (Natural Circulation) Systems The less dense fluid rises above a more denser fluid. This phenomenon can be utilized to create fluid motion in a collector. As water is heated in the collector, it rises to the tank, and the colder water in the tank moves to the bottom of the collector. Solar Energy Systems/ J.P. Agrawal / M2_5- page 2

Natural Circulation Systems… Solar Energy Systems/ J.P. Agrawal / M2_5- page 3 PFlow : Flow pressure drop fluid loop, due to friction in tubings and joints PBouyant : Bouyant force “pressure” in the Caused by the differing fluid densities of the hot and cold legs of the fluid loop. H: Height of the legs L: Height of the collector (x): local collector fluid density Stor : tank fluid density Out : collector outlet fluid density Where the pressure drop in the loop is: K: Sum of the component loss velocity factors in the fluid loop V: Flow velocity

Compact model tank and collector assembly Solar Energy Systems/ J.P. Agrawal / M2_5- page 4 Generally, the thermosyphon systems do not use a pump, therefore, the plumbing fixtures are selected to be more than normal (one size larger is sufficient) to reduce the friction losses. The hot water system loads change very little during a year. Hence, the tilt angle is made equal to the latitude,  = Latitude angle. Tempwater outlet -Tempwater inlet  8-11C during the middle of a sunny day. To prevent the reverse flow in the night time, the top of the collector must be at least 30 cm below the cold leg fitting on the storage tank., else a check valve must be used. An electric heater is used as the back-up during the cloudy days, located near the top of the tank. This location helps in creating and enhancing the temperature stratification inside the tank. Compact model tank and collector assembly

Example: Determine the “pressure difference” available for a thermo-syphon system with a 1 m high collector and 2 m high legs. The water temperature input to the collector is 25C and the collector output temp is 35C. if the overall system velocity factor K is 15.6, estimate the system flow velocity. Solution: Solar Energy Systems/ J.P. Agrawal / M2_5- page 5

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Forced -Circulation Systems Open loop Closed loop Closed loop with drainback Solar Energy Systems/ J.P. Agrawal / M2_5- page 7 Open Loop The solar loop is at the atmospheric pressure, therefore, the collectors are empty when they are not providing useful heat. Disadvantage: high pumping power is required to pump the water into the collector when it becomes too hot.

Closed Loop The solar loop remains filled with water under pressure. Solar Energy Systems/ J.P. Agrawal / M2_5- page 8 Closed Loop The solar loop remains filled with water under pressure. The pump has to overcome only the resistance of pipes. The solar loop consists of a small 2 gallon tank to allow thermal expansion of water from heating, and a pressure relief valve. Because water always stays in the collector of the system, antifreeze (propylene glycol or ethylene glycol) is required in freezing conditions. During the summer months, the temperature in the collector can become very high, leading to high pressure in the loop. This can cause leaks in the loop unless some fluid is allowed to escape through a pressure-release valve (achieved in the system with closed loop with drainback. Please read more details on page 227 of the text.

Solar Air-Heating Systems Application: space heating and heating in small building complexes Please see Table 5.6 for advantages and disadvantages of air v. liquid heating systems. Mainly, the air systems do not have freezing, pipe corrosion, no leak or pressure problems, require no heat exchanger between the collector-storage and storage build-up loops. Flat-plate collectors are ideally suited for the air heating systems. Solar Energy Systems/ J.P. Agrawal / M2_5- page 9

Please read the details in Chapter 4 of the text. Solar Energy Systems/ J.P. Agrawal / M2_5- page 10 Please read the details in Chapter 4 of the text.

Solar Cooling Systems The seasonal variation of solar energy is extremely well suited to the space-cooling requirements of buildings. The highest solar insolation occurs in the warmest seasons of the year when the demand for comfort cooling and refrigeration are the highest. Solar Energy Systems/ J.P. Agrawal / M2_5- page 11 The cooling system consists of a refrigerant that cycles through major components: compressor, condenser, c) expansion valve and d) evaporator. The refrigerant of choice is Chlorofluorocarbon (CFC) The refrigerant enters the compressor as a cold low-pressure vapor. Compressor pumps the vapor to high pressure. When compressed, the temperature and pressure of the vapor increases. The hot vapor enters condenser coil, where it transfers the heat to the environment (air/water) in the right (condenser) chamber. The hot vapor releases latent heat and converts to liquid. The liquid refrigerant then passes through an expansion valve, pressure reduces and the resulting low-pressure mix of liquid and vapor refrigerant enters the evaporator coil. The refrigerant vaporizes in the evaporator coil by taking the heat from the environment (air/water) in the left (evaporator) chamber.

The efficiency of the refrigeration cycle is calculated as, Solar Energy Systems/ J.P. Agrawal / M2_5- page 12 The efficiency of the refrigeration cycle is calculated as, The basic refrigeration cycle can be used to do heating as well as cooling. Heat pumps have the ability to reverse. The W is provided from the solar thermal system.

Absorption Cooling The mechanical compressor in the conventional vapor compression system is replaced with components that perform the same function but do so thermochemically. A refrigerant like water vapor or lithium bromide (LiBr) from the (thermochemical) generator enters thecondensor under high pressure. The vapor changes state to liquid in the condenser, releases heat to the environment. The condenser and evaporator combination works a in the conventional system. The refrigerant entering the absorber is re-pressurized using heat rather than the compressor. The absorber contains lithium bromide (LiBr) into which the water vapor dissolves. A very small pump shuttles the water_LiBr solution from the absorber to the generator. The heat from the solar thermal pressurizes the dissolved water vapor and drives it out of the solution. The LiBr returns to the absorber. Solar Energy Systems/ J.P. Agrawal / M2_5- page 13

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