Solar Thermal Hydronics

Collector Comparison and Selection

In many cases, solar heating is the most energy effective renewable energy technology and is therefore cost effective as well. Thermal collectors convert solar energy to useful heat about five times as efficiently as PV modules convert it to electricity. According to the US Department of Energy, space heating and cooling make up about 53% of US residential energy usage, with hot water adding another 16%. Since solar heat is relatively easy to collect and heating, cooling and hot water demands represent such a large portion of our energy usage, it is likely solar heating will become more mainstream.


Since the oil embargoes of the 1970s and the Carter administration tax credits, certain types of solar heating systems have gone through several generations of market growth. Solar pool heating systems are a mainstay of the industry, with thousands of established installers. Solar domestic hot water systems are being packaged by dozens of manufacturers, and installations of these nearly plug-and-play systems are on the rise.

Both of these system types are relatively easy to design and install, as each addresses a single heating load and each typically interfaces with automated backup heating systems. Solar pool heating systems usually preheat a boiler or operate independently from the backup boiler. Most packaged solar hot water systems include a storage tank with an electric element that comes on automatically when solar heat is insufficient. Seasonal solar pool heating systems often have the advantage of using low cost unglazed collectors, and domestic hot water systems have the advantage of year-round usage, for annual savings that impact system economics and ROI.

Combination (combi) systems, however, have seen much slower market growth than their simpler cousins. This is due to their inherent complexity, which involves the integration of multiple heating loads in a building and regulating the interaction between solar and backup heating sources. When system technology is packaged in a manner that is easy to specify and install, the market size can be substantial, as Table 1 illustrates. Conversely, when the system learning curve is steep, as in the case of solar space heating systems, the market remains largely untapped.

Solar space heating has great potential to become as popular as pool heating and domestic hot water systems have. For this to happen, we need prepackaged, integrated solutions that address system functions that currently require customization. Expanded federal solar tax credits, state incentives and financing programs are boosting interest in integrated systems. Other advances, such as renewable energy credits (RECs) for solar heat produced (analogous to those enhancing PV system economics in many states), will make solar heating even more affordable. It is likely that demand for integrated solar heating systems will increase, and the possibility of dramatic industry growth is great. Installers who have a good understanding of the technology and its applications should see their businesses grow.

Towards that end, in this article we focus on solar heat collectors—specifically glazed hydronic collectors. For decades, these collectors have proven themselves to be effective at providing hot liquid for a variety of heating applications, including domestic hot water (DHW), hydronic (hot water) space heat, radiant warm floors and year-round heated pools.


Solar collector thermal efficiency can be broadly defined as the fraction of available solar energy converted into useful heat during a known period of time. This value can be calculated—by measurement or modeling—instantaneously, hourly, daily or averaged in other ways. Divide the useful energy delivered by the solar energy available and you get the efficiency, which can be expressed as a decimal fraction or as a percent. It is often referred to by the Greek letter Nu (η).

In sales literature, sometimes the only mention of efficiency is a single number, for example η=0.71. This might lead you to expect that a given collector will always convert 71% of the solar energy into heat. In most cases, however, η refers to the best possible theoretical efficiency for the collector in question, which is also known as the optical efficiency. This is the combined efficiency of the transparent cover and the absorber, and it is identified more properly by its own symbol (ηO). While it may be interesting to compare the optical efficiency of one collector to another, this has little to do with its operational efficiency or heat output when installed.


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