9 Solar Thermal Energy.pdf

Energy Systems Engineering Technology ESET 222 Wind & Solar Energy Winter, 2022 Professor: Arun Hor. Solar Thermal Energy ESET 222: Wind & Solar Energy ( Winter 2020) Solar Energy DEGREE DAYS A degree day is a quantative measure of the heating and cooling needs of buildings based upon daily temperat...

Energy Systems Engineering Technology ESET 222 Wind & Solar Energy Winter, 2022 Professor: Arun Hor. Solar Thermal Energy ESET 222: Wind & Solar Energy ( Winter 2020) Solar Energy DEGREE DAYS A degree day is a quantative measure of the heating and cooling needs of buildings based upon daily temperatures. To calculate the Heating degree days (HDD) for a particular day, find the day's average temperature by adding the day's high and low temperatures and dividing by two. If the number is above the base temperature (say,200C) , there are no heating degree days that day. If the number is less than 20, subtract it from 20 to find the number of heating degree days. For example, if the day's high temperature is 180C and the low is 100C, the average temperature is 140C. 20 minus 14 is 6 heating degree days. Cooling degree days (CDD) are also based on the day's average minus 20. They relate the day's temperature to the energy demands of air conditioning. For example, if the day's high is 40 and the day's low is 20, the day's average is 30. So, 30 minus 20 is 10 cooling degree days. Use the degree day’s calculation to establish how much heating / cooling is required. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Energy DEGREE DAYS Degree days are a good way to keep track of how much demand there has been for energy needed to heat buildings. The colder it is outside, the more degree days and the more energy required to heat buildings. Heating and cooling degree days can be used to relate how much more or less you might spend on heating or air conditioning. Of course you'd have to take into account how well insulated your new home will be in comparison to your old one and the different costs of electricity, gas or heating oil. You could also use records of past heating degree days to see if the money you've spent on insulation, or a newer furnace or air conditioner is paying off. To do this, you'd also need records of past energy use. The heating degree season begins July 1st and the cooling degree day season begins January 1st. The following table gives heating and cooling needs for locations in each of the ten provinces and three territories of Canada. Heating needs in Vancouver are about half than in Winnipeg, although differences between other cities in southern Canada are less dramatic. Cooling needs, on the other hand, differ much more widely across the Country. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Energy DEGREE DAYS ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System The solar thermal power system collects the thermal energy in solar radiation and uses at high or low temperature. The low temperature applications include water and space heating for commercial and residential buildings. Producing electricity using the steam-turbine-driven electrical generator is a high temperature application. In the solar thermal power plant, the solar energy is collected by thousands of suntracking mirrors, called heliostats, that reflect the sun’s energy to a single receiver atop a centrally located tower. The enormous amount of energy focused on the receiver is used to generate high temperature to melt a salt. The hot molten salt is stored in a storage tank, and is used, when needed, to generate steam and drive the turbine generator. After generating the steam, the used molten salt at low temperature is returned to the cold salt storage tank. From here it is pumped to the receiver tower to get heated again for the next thermal cycle. The maximum thermodynamic conversion efficiency that can be theoretically achieved with the hot side temperature Thot and the cold side temperature Tcold is given by the Carnot cycle efficiency, which is as follows: ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System where the temperatures are in absolute scale. The higher the hot side working temperature and lower the cold side exhaust temperature, the higher the plant efficiency of converting the captured solar energy into electricity. video ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal Collectors A solar thermal collector is a solar collector designed to collect heat by absorbing sunlight. A collector is a device for converting the energy in sunlight, or solar radiation, into a more usable or storable form. TYPES: Solar collectors fall into two general categories: non-concentrating and concentrating. In the nonconcentrating type, the collector area is the same as the absorber area (i.e., the area absorbing the radiation). In these types the whole solar panel absorbs the light. Flat-plate and evacuated-tube solar collectors are used to collect heat for space heating, domestic hot water or cooling with an absorption chiller. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal Collectors Energy Flow in a Solar Collector ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System Flat plate collectors: Flat plate thermal systems for water heating are the most common type. They consist of (1) a dark flat-plate absorber of solar energy, (2) a transparent cover that allows solar energy to pass through but reduces heat losses, (3) a heat-transport fluid (air, antifreeze or water) to remove heat from the absorber, and (4) a heat insulating backing. This may be achieved directly or through a heat exchanger. Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System The heat is transferred to liquid passing through pipes attached to the absorber plate. There are numerous versions of flat plates available on the market. The essence of the system is that energy from the sun is absorbed by a flat sheet of material, usually some sort of metal, which is covered with a black coating. Water circulates through piping which is attached to this sheet, and the heat is simply transferred to the water. The warm water is stored in a geyser and circulates through the collector. The absorber is housed in a frame and covered by a transparent material to allow the radiation from the sun through and offer some protection against heat loss due to natural elements. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System Evacuated Tube Collectors: An evacuated tube collector consists of a manifold and glass tubes. An evacuated tube is a double walled glass tube which is sealed at one end. The space between the two layers of glass is evacuated, thus creating a vacuum. This reduces the loss of heat to adverse ambient conditions. The inner wall of the tube is covered with selective coating to attract and trap the energy of the sun. Heat trapped in the tube is transferred to a manifold, water then runs through the manifold and heats up and is then transferred to the solar geyser. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System Evacuated Tube Collector In ETCs the heat can either be gathered by means of a solar collector fluid flowing through the absorber as in flat plate collectors or it can be collected by means of the heat pipe principle. In a heat pipe there is only a small amount of fluid sealed inside each evacuated tube. The energy transfer takes place in four steps: 1. This fluid is evaporated by the solar radiation. 2. The vapor rises to the top where it meets a (colder) pipe where a liquid flows through. 3. The vapour is condensed thus transferring the latent heat to the liquid in the top pipe. 4. The condensed fluid in the evacuated tubes runs back to the bottom of the tube where the process can start again. In practice the process is continuous and not stepwise. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System Evacuated Tube Collectors The tubes are made from low emissivity borosilicate glass (glass with a very low iron content that has superior durability and heat resistance ) with an all-glass seal and they employ selective coating. This produces greater thermal efficiency in bright sunshine and also produces high efficiency in overcast or diffuse sunlight conditions. Further, the tubes are evacuated and have a barium getter (vacuum indicator) which changes color from silver to white if a tube’s vacuum has been compromised. An examination of the tubes shows that the outside is actually 2 layers of glass and a vacuum has been created between them. A good way to demonstrate this would be to fill an empty tube with very hot water and notice that it does not even get warm as you hold the tube in your hands. That is because of the “thermos effect” created by having a vacuum between the layers of glass. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System Evacuated Tube Collectors: While evacuated tube technology clearly surpasses flat panels for nearly all water heating applications, the advantages are truly dramatic when used for solar air conditioning, heating or commercial process. That’s because evacuated tube heat pipe collectors can more easily attain the higher temperatures needed, they can collect and retain heat even when it is very cold outside. The evacuated solar thermal collector system uses evacuated tubes (glass-glass seal), copper headers, copper heat pipes, aluminum casing, glass wool insulation, and an all-stainless steel frame. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System Evacuated Tube Collectors The header is made from copper which makes for excellent heat transfer and is corrosion resistant and allows all connections to be brazed rather than soldered. The manifold uses a powder-coated allaluminum casing for durability, structural integrity and light weight. The manifold is packed with glass wool insulation and is sealed with special UV stabilized silicone rubber that can withstand temperatures of up to 250 degrees C. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System Evacuated Tube Collectors Heat Pipe: A heat pipe allows for rapid heat transfer. The heat pipe itself is a copper tube that maintains a vacuum and contains a small amount of liquid. The low pressure (vacuum) in the copper pipes means that the liquid boils at a low temperature (about 30C/ 86F), turning to steam and rushing up to the heat of the heat pipe, carrying heat. It dumps the heat (to the glycol solution running through the header), condenses and runs back down to repeat the process. Normal water pipe copper (as used for the header) is not suitable for the heat pipe because over time oxygen and other trace element leach out, forming an air pocket in the top of the heat pipe. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System Solar Collector Efficiency A simple way to calculate the efficiency is to use following equation: Where, The collector power output can be calculated as: Where, ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System Solar Collector Efficiency Comparison of FPC and ETC: One evacuated tube collector and two flat plate collectors are listed below with the corresponding efficiencies assuming a solar radiation of 1000 W/m2. Area of a Collector: The aperture area is the area of the collector which actually takes part in the energy conversion. This is the area normally used for the collector efficiency. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System 1. Collector Temperature Sensor 2. Solar Collector Panels 3. Solar Pump Station 4. Fill & Drain Valve for system draining 5. Solar Tank / Heat Exchanger 6. Solar Control system 7. Expansion Tank ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System Pool or Unglazed: This type of collector is much like a flat-plate collector, except that it has no glazing or transparent cover. It is used extensively for pool heating, as it works quite well when the desired output temperature is near the ambient temperature (that is, when it's warm outside). As the ambient temperature gets cooler, these collectors become extremely ineffective. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System Solar Air Collector Solar air heating is a solar thermal technology in which the energy from the sun, solar insolation, is captured by an absorbing medium and used to heat air. Air collectors can be used in any south facing window for heating. The collector has an airtight and insulated metal frame and a black metal plate for absorbing heat. Solar radiation heats the black panel that, in turn, heats the air in the collector. Solar electrical power blower pulls air from the room through the collector, and blows it back into the room. Solar air collectors can be commonly divided into two categories: Unglazed Air Collectors: Used primarily to heat ambient air in commercial, industrial, agriculture and process applications. Glazed Solar Collectors: Recirculating types that are usually used for space heating. ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System Example: If the solar radiation is assumed to be 1000W/m2, what would be the temperature increase (ΔT) in the water tank (see fig. below) after 5 hours of exposure (consider two panels with the area of each solar panel being 2.37 m2, quantity of water in the solar tank is 80 US gallons, efficiency of solar panel is 70%, and the ambient temperature is 200C). What is the temperature of water in the tank? ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal System Example: If the solar radiation is assumed to be 1000W/m2, what would be the temperature increase (ΔT) in the water tank (see fig. below) after 5 hours of exposure (consider two panels with the area of each solar panel being 2.37 m2, quantity of water in the solar tank is 80 US gallons, efficiency of solar panel is 70%, and the ambient temperature is 200C). Solution: We have, area per solar panel = 2.37 m 2 Quantity of water in solar tank = 80 US gallons = 80 x 8.35 Ib = 668 Ib. (1 US gallon = 8.35 Ib) Solar Radiation, Q = 1000 W/m 2 ; Efficiency of solar panel = 70% For two panels, P = 1000 x 2.37 x 2 x0.7 = 3318 Watts (1kW = 1 kJ/sec) P = 3.32 kW = 3.32 kJ/Sec x 3600 sec/hr x 5 hrs = 59760 kJ P = 59760 kJ x 0.95 =56772 Btu (1 kJ = 0.95 Btu) (1Btu is raising1 Ib water to 1 0 F) So, ΔT = 56772 Btu / 668 Ib = 85 0 F = 29.4 0 C So, the temperature increase in the water tank after 5 hours = 29.40C ESET 222: Wind & Solar Energy ( Winter 2020) Solar Thermal Energy