GLUMAC - Engineers for a sustainable future

Mitchell Dec, Portland Energy Analyst, Glumac Energy Services and Geoff Winslow, Portland Senior Associate

Solar thermal systems use the sun to heat either water or other heat-transfer fluid in a collector. The heated fluid is then held in a storage tank until needed. A conventional system may require additional heating from a supplementary heater.

Along the West Coast, the simple payback we have seen ranges from two to ten years depending upon contractor pricing and the rebates available at the project location.

Background

The use of solar hot water heaters has a surprisingly long history. In 1891, Clarence Kemp of Baltimore, Maryland patented the world's first commercial solar water heater. In the late 1800s, before oil and gas became available in the West, more than one third of Pasadena, California residents had solar hot water systems.

The energy crisis of the 1970s created renewed interest in solar hot water as a way to conserve fossil fuels, leading to federal and state tax breaks. However, as energy prices dropped in the early 1980s, these incentives expired and the market for solar water heaters collapsed.

Fortunately, interest in solar hot water continued in other countries, so the technology steadily improved. As a result, today™s solar hot water systems produce a net profit for system owners in less than ten years. Once again, an increase in energy costs has renewed interest in solar hot water.

How it works

Solar thermal systems, unlike Photovoltaics (PV), are comparatively low-technology systems that rely on direct transfer of radiant solar energy into heat through collectors usually consisting of:

• A highly conductive material such as coated copper designed to maximize absorption of radiant energy
• A glazing system covering the collecting surface and designed to minimize re-radiation of energy to the environment
• Insulation designed to reduce energy loss by conduction and other processes either with traditional insulating materials or a vacuum

The collected heat is typically transported using water or heat transfer fluid to move the energy from the collector to a point of use.

System components

System components will vary depending on system design, but the most common major elements are:

Solar collectors - These devices collect, absorb, and transfer solar energy to a working fluid, such as water or air. The most common types are:

• Unglazed, flat-plate collectors - Simple solar hot water collectors, constructed from plastic without the glass covering of a glazed, flat-plate collector. Inside the collector box are a series of copper tubes and fins, painted black to collect the sun™s energy. Because they are not insulated, these collectors are used in applications (such as heating for an outdoor pool) that do not require solar hot water heating during the winter months. They are ideal for such applications because of the low first costs, and significant hot water production during the summer months.
  
  Suggested manufacturer: Heliocol
• Glazed, flat-plate collectors “ Very similar to unglazed, flat-plate collectors except that they are constructed to provide additional hot water during the winter months. The box is made from metal, typically aluminum or steel, and is insulated to allow for collecting the sun™s energy during the winter months, while mitigating heat loss from low ambient temperature. Additionally, a piece of tempered low-iron glass helps to attract UV energy even on a cloudy day.
  
  Suggested manufacturers: Heliodyne, Alternate Energy Technologies, Mr. Sun Solar, SunEarth
• Evacuate tube collectors “ A newer technology originally developed in Canada for climates that are typically cold, but still have significant annual solar energy available. Since flat-plate collectors are more susceptible to losses in efficiency during very cold days, the evacuated tube collector encloses the tubes and fins within a vacuum seal, providing a very efficient system under cold conditions. However, its efficiency on average days is lower compared to flat-plate collectors, resulting in a reduced annual production.
  
  Suggested manufacturer: Focus Technology (Apricus), Thermomax
• Storage Tanks - These tanks temporarily store the solar heated water. In a centralized domestic hot water plant, solar heated water production serves as a preheat system. Energy savings occur even when the solar hot water system only increases the water temperature by a fraction of the total desired temperature rise. The storage tank should range from 1.25 to 2 gallons of capacity per sf of collector area. For an office application, the storage capacity should be lower“1.25 gallons per sf. For a commercial building with significant hot water use expected, the storage capacity should be sized for at least 1.5 gallons per sf of collector area. This will allow the owner to consider adding additional collectors in the future.

Heat Exchangers - Most building codes in the United States require that the solar collectors use a heat exchanging fluid, and transfer the heat to the domestic hot water loop via one of the following types of heat exchangers:

• Plate-and-frame “ External to the storage tank.
• Double-wall coil “ Integral to the storage tank.

In general, the storage tank with the internal heat exchanger results in a reduced system cost, with higher peak efficiencies, when compared to a system with a plate-and-frame heat exchanger. However, the heat exchanger fluid must be food-grade in case cross-contamination of the domestic hot water supply occurs.

Other components: expansion tanks, piping, pumps, solar controller, tracking devices.

When To Use This Technology

In any building where there is a predictable, continuous hot water load, solar hot water should be considered. For most systems, glazed, flat-plate collectors are recommended.

When selecting an evacuated tube collector for a project in the United States, the climate should be similar to Minnesota rather than the climate(s) on the West Coast. Additional consideration should be made in regard to the manufacturer used. The vacuum seal can wear down due to significant changes from very cold temperatures to hot temperatures. If the seal fails, then the increase in efficiency during the winter months can be negated, and the collector can be prone to leaks.

Nearly all projects can incorporate solar hot water. However, the following types of projects will offset the greatest percentage of their total energy: multi-family residential, pools, spas, kitchens, and other projects with a large process hot water use.

Economic Benefits

Solar thermal systems are economically feasible due to the following factors:

• Advances in collectors and heat exchangers
• Government and utility rebates and incentives
• Rising energy costs
• Estimated costs and payback

The cost and payback period of a solar hot water system will vary depending on several factors:

• Size of system
• Rebates or incentives
• Electricity and natural gas rates
• Local market conditions for materials and labor

The following is an example of the cost effectiveness for solar hot water heating on a Southern California project:

• System was sized for 2,600 sf of collector area
• Hot water storage capacity of 4,000 gallons
• Preliminary cost estimate = $183,000
• Federal EPAct Tax Credit net-present value (after accounting for accelerated depreciation) estimated to be nearly $70,000
• Estimated annual energy cost savings nearly $22,500 annually
• Five-year simple payback after Federal EPAct Tax Credit

The following is an example of the cost effectiveness for solar hot water heating on a project in Portland, Oregon:

• System was sized for 3,000 sf of collector area
• Hot water storage capacity of 4,500 gallons
• Preliminary cost estimate = $260,000
• Estimated Energy Trust Rebate = $51,000
• Estimated Oregon BETC Renewable Energy Tax Credit = $63,750
• Federal EPAct Tax Credit net-present value (after accounting for accelerated depreciation) estimated to be nearly $117,000
• Estimated annual energy cost savings nearly $10,200 annually
• Three-year simple payback after all rebates and tax credits

With the recent increases in utility and government incentives for solar programs, the payback period for solar domestic hot water heating systems is approximately three to eight years in Oregon and California. For other states, where the incentives or solar heat gain is not as lucrative, the payback is closer to eight to twelve years. This payback period makes solar domestic hot water heating worthwhile for consideration on all projects, especially those with large hot water loads, such as multi-family residential, restaurant, and pool heating projects.

Combining With Low Flow Fixtures To Maximize LEED^®Points

Solar hot water systems can always be reduced in capacity needs through the use of low flow fixtures. The following fixtures can reduce the hot water demand on most facilities:

• Lavatory sinks
• Kitchen sinks
• Low flow showerheads

The 1992 EPAct maximum allowable flow rate for the fixtures, above, is 2.5 gallons per minute (gpm). Using aerators can bring lavatory sinks down to 0.5 or 1.0 gpm, and kitchen sinks down to as low as 1.2 gpm. Low flow showerheads need to be selected carefully. By selecting a showerhead at 2.0 gpm, or less, the hot water load can be greatly reduced. Therefore, the system size of the solar hot water array can be reduced to make the system as cost effective as possible.

Using low flow fixtures and solar hot water heating will contribute LEED^® points in the following categories:

• Water Efficiency (WE) Credit 3: Can help obtain the goal of 40%+ water use reduction for the 2 points under WE Credit 3, as well as 1 Innovation in Design point.
• Energy & Atmosphere (EA) Credit 1: Low flow hot water using fixtures will reduce the energy consumed for total gallons of hot water heated annually. The solar domestic hot water will contribute to the total percentage of energy cost savings to obtain additional points for EA Credit 1.
• Energy & Atmosphere (EA) Credit 2: A solar hot water heating system should be capable of offsetting between 40% and 60% of a building™s total hot water energy required annually. Therefore, any building that attributes 5%, or more, of its annual energy cost to hot water heating can obtain at least 1 point for on-site renewable energy.