Solar thermal

by Dan Gretsch

According to the U.S. Department of Energy, 47% of the average home's energy use is for heat—either domestic hot water or space heating. Using specially designed collectors, solar hot water (SHW) systems collect the sun's heat and store it in water. In turn, this stored energy is tapped for domestic uses like showers, laundry, dish washing, or even heating your home.

Investing in Solar Energy

Installing a solar hot water system represents one of the best renewable energy (RE) investments that the average homeowner can make. While wind and microhydro can have a lower cost per KWH equivalent of energy generated, only a small percentage of Americans live where they can install either of these technologies.

On the other hand, most home sites in the United States have at least some access to direct sunlight. Solar electricity— converting sunlight into electricity—is very popular. But compared to solar hot water collectors, photovoltaic modules are comparatively inefficient and expensive. The efficiency range for crystalline PV modules converting sunlight into electricity is about 12% to 19%, and recouping an investment in a grid-tied residential PV system, even with incentives, can take between ten and twenty years. In contrast, for domestic water heating applications, SHW collectors convert about 60% of the energy in sunlight into hot water, producing up to about six times more usable energy per dollar invested. In locations with favorable incentives, SHW systems can recoup their costs in about three to seven years.

Even without any available financial incentives, a professionally installed SHW system with 64 square feet of collector area can produce the equivalent of about 20 KWH per day at an installed cost of $5,000 to $8,000. If you wanted to generate that amount of energy with PV, a system size in the range of 3.5 KW would be required, costing approximately $35,000.

Despite the obvious advantages, there has only been a modest increase in the number of SHW installations over the last several years compared to the significant growth in installed PV systems. With rising fuel costs and increasing interest in reducing carbon emissions, economic interests alone should be driving more significant growth in the solar thermal market than we are seeing. So what's the holdup?

Solar Hot Water Simplified

Solar hot water system installers are faced with myriad engineering decisions. Every home is different, and the optimal system type depends on a wide range of variables, including the number of people in the home, how efficiently water is used in the household, the average daily peak sun-hours available at the site, potential shading from nearby trees or buildings, wintertime ambient temperatures, and the location and space available for collectors and hot water storage.

Once the optimal system type—open loop, drainback, or glycol—is chosen, the installer needs to determine collector type (flat plate or evacuated tube), collector sizing, flow rates, pump performance, heat exchanger sizing, valve type, wiring, mounting, equipment sourcing—and decide how to integrate all the components together. Despite comparatively modest system costs compared to PV, technically complex system design and installation is one reason that many potential installers shy away from the SHW field.

Three common product packages in the European SHW market have rapidly streamlined solar thermal installations overseas and are gaining popularity in the United States.

• Component packages

• Pre-assembled pump stations

• Integrated pump and heat exchanger units

The goal of each approach is to simplify system design, planning, and engineering, and to minimize system installation time and complexity.

Component Packages

To ease the burden of sourcing all of the different components required for an SHW installation, several companies offer packages of unassembled components. This eliminates some sourcing headaches but doesn't necessarily address the range of complex engineering issues that installers face. For example, as a situation dictates, you may need a larger heat exchanger or a different pump capacity. And, once you have addressed the design issues, you still have to assemble all of the individual components. A typical component package contains:

• Solar collectors

• Storage tank with heat exchanger

• Differential temperature controller

• Fluid circulation components

• Circulation pumps

• Installation manual

• Mounting hardware

A pump station manufactured by Oventrop.

A pump station manufactured by Oventrop.

Oventrop Pumpstation
The Solarnetix pump station. The front insulation is removed to show the components.

While component packages reduce the time required to source individual system parts from various manufacturers or suppliers, unassembled packages rarely reduce the installation time. With all of the individual components in hand, it will still take an experienced crew of two to three people two full days to assemble and install a system. Knowledgeable do-it-yourselfers will probably require twice that time. Unlike pros, they won't have a van or truck stocked with extra fittings, and will usually have to make multiple trips to the hardware store for elbows, couplings, and the like that aren't included in packaged systems but are necessary for site-specific installation. Then comes dry-fitting the pieces, making connections, pressure checking, repairing any leaks, rechecking pressure, and then starting up the system.

While unassembled component packages give the system installer a leg up, solutions with greater pre-integration and preassembly are available that can significantly reduce installation time and complexity.

Pump Stations

Pump stations were relatively common in the U.S. solar thermal industry in the 1980s, but virtually disappeared until several years ago, when companies in Europe began offering integrated pump packages (also referred to as circulation stations) for use with specialty heat-exchange tanks. This approach was a giant step toward reducing the complexity of an installation. With a variety of insulated, pre-plumbed components, these systems reduce the time required for the sourcing, connecting, and pressure-checking steps of an installation.

Pump stations do reduce installation time and complexity, but they don't cover all of the bases. Some stations include controls; others do not. Most are designed to integrate with specialized storage tanks with an integral heat exchanger. The cost of these specialty tanks alone can run $600 more (plus shipping) than a standard, locally purchased hot water tank.

EnerWorks manufactures integrated pump, control, heat exchanger, and storage tank systems.

Pre-Packaged Pump & Heat Exchangers

Pre-packaged systems—which integrate heat exchangers into the pump packages—work with standard hot water tanks. They include heat exchangers, pumps, isolation valves, controls, system drains, pressure relief valves, fill valves, pressure gauges, and check valves, all in a single package. Many are compact in design, which enables easy installation in relatively tight places. Each comes with virtually all of the special plumbing components pre-assembled to facilitate on-site installation.

While all of the available integrated systems work, the buyer still needs to pay careful attention to their suitability for a particular situation, including cost. Every system, integrated or not, has to address the questions of flow through the collectors and the transfer of heat to the storage tank. For example, some systems rely on thermosyphoning, convection that circulates liquid without requiring a pump. In these systems, although the cost and complexity are reduced, the slower flow reduces overall system performance. Different manufacturers also use different-sized heat exchangers. Systems that use smaller-capacity heat exchangers will be cheaper, but those that use larger exchangers will allow pumps to run less for the same amount of heat exchange. The size and type of pumps also influence system performance and cost—smaller pumps keep system costs lower but also reduce the flow and efficiency of the heat transfer. Cast-iron pumps are less expensive, but corrode sooner than bronze pumps when subjected to degraded glycol or dissolved oxygen that is present in the heat-transfer fluid.

Comparative Shopping

While SHW packages and integrated component assemblies can greatly simplify system installation, it is still up to the installer—or the do-it-yourselfer—to choose the correct equipment to suit a specific customer and site. The first thing a buyer should insist on is that components are certified by the Solar Rating and Certification Corporation (see Access). Shop around and understand the relative merits of each system—one pump or two; bronze, stainless, or cast-iron pumps; larger or smaller heat exchanger; Underwriters Laboratories certified controls; particulate filtration—enough differences exist between systems to require a consumer to tread carefully.

The expense of on-site assembly and the need to guarantee long-term system performance may make pre-assembled solar hot water systems the wave of the future. With a number of pre-assembled system manufacturers in the market, there has never been a better time to install a solar hot water system on your home.

Heat exchanger, pump, and control package from Oventrop.

Access

Dan Gretsch is a registered professional engineer with 15 years of experience working on energy efficiency projects. He has managed industrial energy efficiency projects in the United States, Europe, and Central and South America. He is vice president of engineering for SolarH2Ot (www.solarhotusa.com) in Cary, North Carolina.

Solar Rating & Certification Corporation • www.solar-rating.org System Packages, Pump Stations & Integrated Heat Exchanger Units:

AAA Solar • www.aaasolar.com Alternate Energy Technologies • www.aetsolar.com Alternative Energy Store • www.altenergystore.com Apricus • www.apricus.com

Butler Sun Solutions • www.butlersunsolutions.com

Caleffi • www.caleffi.us

Heliodyne • www.heliodyne.com

Oventrop • www.oventrop-na.com

Purist Energy • www.puristenergy.com

SolarDirect • www.solardirect.com

Solar Energy-4U • www.solarenergy-4u.com

Solargenix Energy • www.solargenix.com

SolarH2Ot • www.solarhotusa.com

Solarnetix • www.solarnetix.com

Stiebel Eltron • www.stiebel-eltron-usa.com

Viessmann • www.viessmann-us.com

www.stecasolar.com

Steca PF 166

Solar Fridge / Freezer

■ Automatic switch on 12 V or 24 V system voltage

■ Unit runs on a single 70 W module in most climates

■ Digital temperature setting and display

■ Power outage display

■ Fast cooling due to compressor speed control

■ Patented Stop-Frost system to reduce condensation

Steca PF 166

Solar Fridge / Freezer

■ Automatic switch on 12 V or 24 V system voltage

■ Unit runs on a single 70 W module in most climates

■ Digital temperature setting and display

■ Power outage display

■ Fast cooling due to compressor speed control

■ Patented Stop-Frost system to reduce condensation

Steca GmbH | Germany | [email protected]

As per your request.

T-Rex

Charge Controller 200 Volts PV Voc 100 Amps output All the T80 features

SOC Energy Meter Wireless Remote Optimal MPPT

T80 Also available

Apollo o www.ApolloSolar.com

Solar

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