Storage Strategies for Comercial SHW Systems

The mention of the word solar rarely conjures up images of the storage tanks, heat exchangers, pumps and cycling fluids that comprise the typical commercial solar water heating (SWH) system. Large tanks full of solar-heated water do not quite gibe with the public perception of the solar industry, despite the close kinship between PV and SWH technologies. Solar heating professionals, however, recognize that storage is a fundamental element of most SWH systems. As is the case with system design (see “Solar Thermal System Design,” December/ January 2012, SolarPro magazine) and system commissioning (see “Turning on the Heat,” June/July 2011, SolarPro magazine), devising a strong storage strategy is central not only to the success of an individual installation, but also to the ascendency of the solar heating industry itself.

Delving into the intricacies of SWH storage can be an onerous exercise, due to the wide variety of available storage options and the infinite variations in how storage tanks can be integrated with other equipment and systems in use at the client’s facility. However, for solar heating professionals, finding the perfect storage solution for a given project is an exciting challenge. If the right solution is put in place— and all other system elements are designed and installed correctly—the result is an efficient, well-functioning SWH system that will offset utility costs for decades. In this article, we dive into the somewhat esoteric world of SWH storage by exploring the many options available, examining some factors that go into determining a storage strategy and addressing installation considerations for commercial SWH storage.

Determining a Storage Strategy

In the face of widespread public unfamiliarity with the technology, solar heating professionals often borrow nomenclature from the PV world to explain their work, describing the storage tank as a “battery.” Essentially, all solar heating is “off grid,” and the tank does indeed serve as the battery, where the energy produced by the collectors is stored for later use. While there are many different phase-change materials and mediums that can be used to store the energy generated by a solar heating system, in this article we focus on water as the storage medium because that is what more than 98% of SWH installations currently use. Solar heating systems typically include one or more water tanks that form the backbone of the system’s storage capacity. (In this article we refer to the “tank” rather than calling out “tank or tanks” at each reference.)

Does the System Really Need Storage?

The first question designers must ponder is whether storage is necessary at all. Only after settling this decision should designers begin wrestling with weighty matters such as storage capacity, number of tanks, tank type and location in the client’s facility. Not every SWH system requires a tank. Solar pool heating systems, for example, transfer heat directly to the pool water, effectively turning the pool itself into the storage tank.

Incorporating storage into the system allows for the use of solar energy during solar-deficient times, which can be as predictable as nightfall or a solar eclipse, or as unpredictable as heavy cloud cover or midday rainstorms. For a SWH storage tank to be a worthwhile investment, the system must produce more heat than is used when solar energy is present. This is almost always the case with intermittent loads that require heat.

To determine whether SWH storage is necessary, begin by asking these questions:

  • Is there a constant load throughout the day, 365 days per year?
  • Will the solar array produce less than the constant load on the hottest, longest day of the year?

If the answer to both of these questions is “yes,” then the system may not benefit from storage. Removing storage from the equation can greatly reduce system cost and increase the project’s ROI. Examples of systems that are often installed without storage are process heat systems, swimming pool heating systems and district heating systems. Be careful when specifying solar heating systems that do not include storage. They are vulnerable to failure and a severe lack of production if the load is not present, even for a short period of time.

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