Because it is rarely cost-effective (or technically feasible) to size a SHW system to cover 100% of heating load, SHW requires auxiliary (back-up) heating, which is commonly served by the existing fossil fuel system. To this end, DOE reports that combi-systems should provide a solar fraction of 40% to 80% of a home’s heating needs.54 Within Massachusetts, most combi-systems are designed to meet slightly less, between 30% and 50% of the DWH and space heating load.
The design of SHW heating systems depends upon the local climate as well as building readiness, customer aesthetics, and customer budget. SHW configurations typically include solar collectors, water storage tanks, piping, insulation, valves, gauges, and fittings. Depending upon the complexity of the installation, they may also require heat exchangers, pumps, and controllers. Because of Massachusetts’ cold climate, systems must also provide freeze protection during the winter. With this in mind, the following briefly discusses the basic design and installation of SHW systems, considering in particular SHW collectors, controls and pumps, freeze protection, and the integration of combi-systems into the existing heat distribution system.
4.2.1.1 Solar Collectors
Solar collectors concentrate the sun’s energy to heat transfer fluid, turning solar energy into usable heat for space and water heating. Typically installed on building rooftops (though sometimes ground- mounted), each collector panel consists of a network of pipes filled with water or glycol (heat-transfer fluid), which is distributed across the panel surface and heated by the sun. Collector sizes vary, though they typically measure four by eight feet in size.55 Two types of solar collectors –flat plate (glazed and unglazed) and evacuated tubes – are in use in Massachusetts.
Flat plate collectors consist of an absorber plate—a sheet of copper, painted or coated black—bonded to pipes that contain the heat-transfer fluid. The pipes and copper are enclosed in an insulated metal frame and, in glazed collectors, topped with a sheet of glass (glazing) to protect the absorber plate and create an insulating air space.56 In unglazed flat-plate collectors, commonly used for swimming pool heating, the insulating properties of the glazing are not in use, resulting in decreased efficiencies of the panels at lower air temperatures. For applications outside of pool heating, unglazed collectors are generally not appropriate for heating in the Northeast.
In evacuated tube collectors, each absorber plate is contained within a glass tube from which all air has been evacuated – creating a vacuum. Because a vacuum is a better insulator than air, these collectors have much better heat retention than the glazed design of flat-plate collectors – especially in cold climates.57 They also tend to be more expensive than flat plate collectors.
39 Figure 16: Evacuated Tube and Glazed Solar Collectors (Source: RETScreen, 2005)
Depending upon panel design and efficiency, the temperature of the heat-transfer fluid will vary. For example, on a typical summer day (sunny and warm), the fluid in the solar collectors reach 140°F to 180°F (60°C-80°C). On a clear winter day (sunny and cold), it can reach 120°F to 150°F (50°C-65°C). When it´s cloudy and warm, collectors can reach 70°F to 90°F (20°C-30°C), and when it´s cloudy and cold, 50°F to 60°F (10°C-15°C).58 Massachusetts installers report that panels typically deliver heat around 120°F in the New England winter. As long as the temperature in the fluid in the collector is greater than that of the incoming cold water, then the SHW system is saving energy for the user.59
4.2.1.2 Controls and Pumps
After the heat-transfer fluid is heated in the panel, the heat must be transferred from the panels to the building’s hot water system. In active systems, pumps and other mechanical systems move the fluid from the panels to hot water tanks.v In this case, controls and temperature gauges sense when fluid in the collector is hotter than the water in the storage tank, which turns the pump on and circulates the fluid – moving heat from the collector to the water storage tank. When the tank is hotter than the collector, the pump is turned off, thus preventing heat loss through the solar collector. This function is usually performed by a differential thermostat control system, which compares heat sensor readings from the storage tank and collectors and switches on the pump accordingly.
4.2.1.3 Stagnation and Freeze Protection Design
Massachusetts is subject to cold winters, and solar heating systems must be designed to withstand freezing conditions. Because water expands when it freezes, the pipes of a SHW system will burst when temperatures drop, causing potentially significant and costly damage to the system. As a result, SHW systems in Massachusetts are typically designed to use glycol (antifreeze) or a drainback mechanism – both of which protect the system from damage during freezing conditions.
Similar to antifreeze in cars, glycol systems prevent damage in SHW systems by using a mix of water and antifreeze as the heat transfer fluid.w The use of glycol requires a closed loop (indirect) system design,
v
In contrast to active (forced-circulation) systems, passive systems do not use mechanical systems to move the heat transfer fluid, relying instead on thermodynamic properties of the system. In this case, a third system design – the passive
thermosyphon system – could be employed, though it is not currently in wide use in Massachusetts.
w
However, solar hot water systems use nontoxic propylene glycol instead of highly toxic ethylene glycol used in most
automobiles. For more information, see: Marken, C. (February & March 2011). Solar Hot Water: System Types and Applications. Home Power. 141. Retrieved from http://homepower.com/view/?file=HP141_pg48_Marken.
40 meaning that the glycol never mixes with the hot water used in a building. Instead, as the glycol heats up in the collector, heat is transferred from the glycol to the hot water tank through a heat exchanger, which usually consists of a copper coil located inside the hot water tank (see Figure 17 below).
Closed loop drainback systems, on the other hand, typically use water as the heat transfer medium.x To protect against freezing, drainback systems are designed so that water drains back to a reservoir during cold conditions, leaving the collectors and piping filled with air (see Figure 17 below). To ensure proper operation, the collectors and piping must be sloped at an appropriate angle so that the system fully drains.60
Figure 17: Closed-Loop Drainback and Glycol (antifreeze) SHW Systems (Patterson, n.d.)
In addition to freeze protection, Massachusetts installations must also provide safeguards against stagnation, which is a condition in which heat transfer fluids boil off in the collector due to prolonged solar exposure with no cooling flow. In systems using glycol, the glycol will break down under high temperatures. Systems may be protected against stagnation by using advanced controllers (with thermal cycling functions or a vacation/holiday mode), which keep heat transfer fluids from overheating. Alternately, heat dump radiators or properly sized expansion tanks and relief valves can protect the system by allowing the system to dump (dissipate) heat that is not needed.
4.2.1.4 Combi-systems and Space Heating
Figure 18 below illustrates a typical solar combi-system design. In this setup, the SHW system heats water that is stored in the hot water tank. The water then travels from the tank through an auxiliary heating system (i.e. a boiler), which heats the water to higher temperatures (if necessary). The hot water is then circulated throughout the building via the existing heat distribution system.
x
41 Figure 18: Combisystem Design for Solar Space and Water Heating (Drueck, H., Heidemann, W. & Mueller-Steinhagen, H.,
2004)
For the solar combi-system to operate effectively in Massachusetts, it must be integrated into a low- temperature heat distribution system – like radiant floor or radiant baseboard heating. In low- temperature heat distribution systems, the circulating water is close to room temperature, which is ideal for SHW. For example, installers report that typical SHW systems in Massachusetts can heat water to temperatures around 120 degrees Fahrenheit during the winter. By contrast, high temperature heat distribution systems (like a traditional radiator distribution system) circulate water at much higher temperatures – between 122°F and 175°F (50°C and 80°C respectively). As a result of high water temperature requirements, SHW is inadequate for space heating employing high temperature heat distribution.