Issue 79 March 2011e-News

n Introductionn Typical Applicationsn Pool Heatingn Domestic Water Heatingn Thermal Power

n System Performancen Cost Effectivenessn Innovative Technologies

n Related Resourcesn Training Highlightsn About e-News

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 pho-tovoltaic (PV) panels with solar water heating or space heating. An-other 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 commer-cial systems to power plants.

Typical Applications

Pool HeatingPerhaps the simplest use of solar thermal energy is solar pool heat-ing. 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 HeatingFor domestic water heating, a typical solar domestic hot water system consists of the solar collectors, a solar storage tank and heat exchang-er, and controls. The medium-temperature collectors used are either flat-panel collectors, or evacuated tube collectors. Evacuated tube col-lectors produce higher temperature water (180-200°F), but are consider-ably 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

Energy Design Resources e-News March 2011

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Solar Thermal:The Sun is at Your Service

Introduction

Figure 1 - Solar Thermal Installation 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 Corporation

Energy Design Resources 2 e-News March 2011

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 exchang-er. The system can be active or passive. Active systems use a small pump to circulate water through the collec-tors. 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 integrat-ed collector storage (ICS) or breadbox systems.

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.

Thermal PowerA 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 environ-mental impacts.

System PerformanceSolar collectors are rated by the Solar Rating and Certifi-cation 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 per-formance 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 evalua-tion 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 lati-tude plus 10 to 15 degrees for winter heating, or to lati-tude minus 10 to 15 degrees for summer heating. Also, for domestic water heating, it is best to size collectors to

Cost Effectiveness

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 prod-uct 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 re-bates are in addition to the Federal Tax Credit for so-lar 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 heat-ing. The CSI Thermal Program provides guidelines for qualifying systems. For this program, the collec-tor 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 ef-fectiveness of solar water heating and solar space heating. The maps can be found at: http://www.nrel.gov/gis/femp.html.

Energy Design Resources 3 e-News March 2011

achieve an annual solar fraction of 60% to 70%, since a greater col-lector area would produce excess energy in the summer months.

Innovative TechnologiesWhile 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 circu-lating 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 manufac-turers even offer power purchase agreements for hybrid systems, providing renewable solar electricity and hot water heating at guar-anteed rates.

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 con-centrate the sun’s energy without the use of cumbersome tracking devices. Products designed to be integrated with existing photovol-taic 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 bro-mide 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

Related Resourcesn Whole Building Design Guide:

http://www.wbdg.org/resources/swheating.php

n Federal Renewable Energy Screening As-sistant: a high-level screening assistant only for feasibility studies

http://apps1.eere.energy.gov/buildings/tools_directory/software.cfm/ID=114/pagename=alpha_list

n The Florida Solar Energy Center has a number of resources, including this practical guide to the design and installation of solar water heating systems

http://www2.fsec.ucf.edu/en/industry/re-sources/solar_thermal/index.htm

n For more information on solar power devel-opments in California, see:

http://www.energy.ca.gov/releases/2010_re-leases/2010-12-15_Approval_palen+rice_NR.html

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

Figure 2 - Rooftop Solar Hot Water System

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

This course will provide an overview of the design, specification, and installation aspects of Solar Water Heating systems for both commercial and residential applications. PG&E will present a wide range of systems from simple residential water heating to combined systems that serve multiple heating loads at commercial facilities.

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

http://www.pge.com/mybusiness/edusafety/training/pec/classes/

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 - 12:00 pm) --Edison CTAC

https://www.sce.com/ECR/EnergyCenterClassSchedule.aspx?ORG=CTAC,OTHER

Towards Zero Energy Buildings

This class will provide an overview of the process for conceiving, analyzing, designing and operating a zero-energy building. Topics will include: different definitions of zero-energy and how they impact building design, energy usage characteristics and analysis methods, and synergistic building systems technologies.

March 17 (Thursday 9:00 am - 11:00 am) --San Diego Energy Resource Center (SDERC)

https://seminars.sdge.com/iebms/coe/coe_p1_all.aspx?cc=coe&oc=05

Energy Design Resources 4 e-News March 2011

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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 mount-ed 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).

Training Highlights

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