e-News #79: Solar Thermal - The Sun is at Your Service
Solar thermal systems have been used for decades for domestic water heating and space heating. They provide a cost-effective alternative for water heating, a major component of residential building energy use. Solar thermal systems are also very effective at pool heating. Newer technologies are available that combine photovoltaic (PV) panels with solar water heating or space heating. Another emerging technology is solar-assisted absorption cooling. Solar thermal collectors can also be used to concentrate the sun's energy and produce power in large-scale installations. Solar thermal energy can be used in a variety of applications, from residential systems to commercial systems to power plants.
Figure 1 - Solar Thermal Installation
(click to enlarge)
The installation of solar thermal panels is shown above. These panels supply a solar domestic hot water system with a 120 gallon storage.
Courtesy DOE/NREL, Industrial Solar Technology
Perhaps the simplest use of solar thermal energy is solar pool heating. Unglazed solar collectors heat water to 10-15°F above the ambient temperature. The heated water can then heat the pool water indirectly, through the use of a heat exchanger. Depending upon solar availability and project requirements, the collector area needed for pool heating is roughly 75% of the area of the pool. The first efficiency step for pools should always be a pool cover. For outdoor pools, approximately 70% of the heat loss is due to evaporation. Wind screens can also be effective at reducing the convective heat loss from the pool's surface.
Domestic Water Heating
For domestic water heating, a typical solar domestic hot water system consists of the solar collectors, a solar storage tank and heat exchanger, and controls. The medium-temperature collectors used are either flat-panel collectors, or evacuated tube collectors. Evacuated tube collectors produce higher temperature water (180-200°F), but are considerably more expensive. The controller typically turns the pump on when the return temperature from the collectors exceeds the temperature at the bottom of the storage tank by 10°F. For water heating, there are four basic system designs: direct and indirect, active and passive. Direct systems heat the domestic water without the use of a heat exchanger. These are only suitable for climates where freezing is not a concern (southern Florida). (Note that water can freeze at temperatures as high as 40°F due to radiation to the night sky.) Indirect systems use a glycol-water mix and transfer heat to the solar storage tank via a heat exchanger. The system can be active or passive. Active systems use a small pump to circulate water through the collectors. It is possible to use a small PV panel to power a DC pump. Passive systems locate the solar storage tank up on the roof and rely on natural convection - the absorbed heat causes the fluid to circulate naturally, without the use of a pump. These systems are also known as integrated collector storage (ICS) or breadbox systems.
Figure 2 - Rooftop Solar Hot Water System
(click to enlarge)
Active solar water heating systems use collectors to capture the sun's energy. A powered circulation system moves fluid between the collectors to a storage tank. The unit featured here is PV powered. When it is sunnier, the PV panel increases the flow rate, corresponding with the extra heat energy being captured.
Courtesy DOE/NREL, Commission on Economic Opportunity
As a rough rule of thumb, approximately 40 square feet of collector area is needed for a family of two, and an additional 8 square feet for each person beyond two. For the solar storage tank, approximately 1.5 to 2 gallons per square foot of collector area is needed.
A common use of solar thermal energy is to concentrate that energy to a central receiver to generate power via a turbine. The California Energy Commission has recently approved 650 MW of solar power, with two plants to be built in Riverside County. Since late August, the CEC has licensed more than 4,100 megawatts of solar power in the California desert. The larger of the two plants will use parabolic trough mirrors to heat a heat transfer fluid to drive steam turbines, generating up to 500 MW of power. One major obstacle for "solar farms" is that they require large amounts of land (as much as several thousand acres), and must be developed to mitigate any environmental impacts.
Solar collectors are rated by the Solar Rating and Certification Corporation (SRCC). Two test procedures are used: OG-100 to rate collectors and OG-300 to rate systems. The system test (OG-300) provides a more useful performance metric since the system as a whole, including controls, storage tanks and heat exchangers, is tested with a standard daily draw pattern.
Several tools are available to predict the energy output of solar thermal systems. For preliminary screening for cost effectiveness of solar hot water heating, use the Federal Renewable Energy Screening Assistant (FRESA) software. The software tool, developed by NREL, assists in evaluation of many renewable technologies including solar hot water, photovoltaics, and wind. The f-chart method is a widely used method that predicts monthly energy output of solar pool heating and water heating systems. F-chart software uses this method and performs life-cycle cost analysis for such systems. RETScreen is another tool available from Natural Resources Canada that predicts monthly output of solar water heating systems or solar pool heating systems. For detailed engineering analysis, the software tool TRNSYS simulates performance of solar thermal systems on an hourly basis.
Design tools are useful in determining what impacts design parameters, such as collector efficiency, collector orientation and sizing have on thermal performance. As a general rule of thumb, it is best to tilt collectors at an angle equal to latitude, for year-round heating, or to latitude plus 10 to 15 degrees for winter heating, or to latitude minus 10 to 15 degrees for summer heating. Also, for domestic water heating, it is best to size collectors to achieve an annual solar fraction of 60% to 70%, since a greater collector area would produce excess energy in the summer months.
|Swimming pool solar water heaters cost between $20 and $40/ft2; unglazed collectors are the least expensive type of solar water heater. Flat plate solar collectors can cost $90 to $120/ft2. Evacuated tube solar collectors can be more expensive, with product costs around $70/ft2. The average residential system costs approximately $6,000 installed, before rebates. Commercial systems carry an installation cost of $103-$145/ft2.
The California Solar Initiative (CSI) program offers rebates up to $1,875 for single-family homes, and up to $500,000 for large commercial projects. These rebates are in addition to the Federal Tax Credit for solar DHW systems. The CSI rebate is dependent upon the predicted thermal output of the system,
|based on location, orientation, shading and other factors. They are cost-effective in most instances, including multi-family buildings, where the landlord typically pays for tenant hot water use. Solar water heating systems are most effective when replacing electric water heating. The CSI Thermal Program provides guidelines for qualifying systems. For this program, the collector area should not exceed 1.25 times the estimated load in gallons per day, and should be certified by the SRCC.
NREL has created a set of maps to show the cost effectiveness of solar water heating and solar space heating. The maps can be found at: http://www.nrel.gov/gis/femp.html.
While solar water heating systems have been available since the 1970s, innovative technologies are emerging. Hybrid systems are available that combine photovoltaic (PV) panels with solar hot water collectors to generate both electricity and hot water. The water circulating underneath the PV panels improves their electrical efficiency by removing heat. Another option is to use the heat behind the solar collectors for space heating. PV/T systems also are available with concentrating collectors. Hybrid solar PV and heating systems are also eligible for incentives under the CSI program. Some manufacturers even offer power purchase agreements for hybrid systems, providing renewable solar electricity and hot water heating at guaranteed rates.
The Florida Solar Energy Center has a number of resources related to solar thermal in their publications database
Concentrating collectors are now available in much smaller sizes for rooftop applications, and can be used for space heating, cooling or process heat. New technologies are being developed that can concentrate the sun's energy without the use of cumbersome tracking devices. Products designed to be integrated with existing photovoltaic panels can amplify the sun's energy by 200% to 300%.
Absorption cooling has been used for over a century to provide space conditioning. Modern systems use lithium bromide as the absorber. With absorption cooling, solar energy heats a dilute lithium bromide solution in the generator. As water vaporizes the water-lithium bromide solution is concentrated, and flows to the condenser. The refrigerant vapor is condensed on the cooling coil, and refrigerant liquid is passed from the condenser to the evaporator. Low pressure in the evaporator forces the liquid refrigerant from the condenser to the evaporator. The low pressure causes the refrigerant to evaporate and flow to the absorber. In the absorber the refrigerant vapor is absorbed by the concentrated lithium bromide solution. Heat from condensation is removed by the cooling water.
High-performance flat plate collectors or evacuated tube collectors are required to heat the water to the necessary temperatures used by the absorption cycle (typically 160°F to 200°F). Products are available for small commercial buildings, and products for residential use are under development. Solar assisted absorption cooling is being demonstrated at the Southern California Energy Resource Center (ERC) in Downey, CA. The concentrating solar collectors will produce about ten tons of cooling.
Another technology that has not been widely used in North America is district solar thermal heating. An array of solar collectors can be mounted on rooftops or nearby land, sized to meet nearly 100% of the annual thermal heating load. In summer, the excess thermal energy is stored in the ground through a series of vertical boreholes or a large, insulated storage tank. Over a period of a couple of seasons, the bore field is fully charged, and can be used to meet hot water heating needs during the winter months. An example of this technology is the Drake Landing Solar Community in Alberta, Canada (http://www.dlsc.ca/index.htm).
California utilities offer outstanding educational opportunities that focus on the design, construction and operation of energy-efficient buildings. Listed here are a few of the many upcoming classes and events; for complete schedules, visit each utility's website.
Solar Water Heating Systems
March 16 (Wednesday, 9:00 am to 4:30 pm) San Francisco--PEC, Also online
April 7 (Thursday, 9:00 am to 4:30 pm) Marina--Marina Airport
April 28 (Thursday, 9:00 am to 4:30 pm) Berkeley--YMCA Teen Center
CSI Contractor Solar Class
Photovoltaic (PV) installers, self-installers, managers, and PV owners will find out about new features and updated information on the California Solar Initiative program; learn how to complete the program forms, take advantage of SCE's rebates on fixed and tracking photovoltaic (solar energy) systems, and interconnect to the grid.
March 17 (Thursday 8:00 am -
Towards Zero Energy Buildings
March 17 (Thursday 9:00 am - 11:00 am) --San Diego Energy Resource Center (SDERC)
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e-News is published by Energy Design Resources (www.energydesignresources.com), an online resource center for information on energy efficiency design practices in California.
Savings By Design (www.savingsbydesign.com) offers design assistance and incentives to design teams and building owners in California to encourage high-performance nonresidential building design and construction.
Energy Design Resources and Savings By Design are funded by California utility customers and administered by Pacific Gas and Electric Company, Sacramento Municipal Utility District, San Diego Gas and Electric, Southern California Edison and Southern California Gas Company, under the auspices of the California Public Utilities Commission.