Batteries of Solar Thermal Energy

By Peter Biondo

“The sun is the source, the system is the battery.” Given the variety of applications for which solar energy can be harnessed, the energy storage principle for a well planned and designed solar thermal system is extremely important for overall system efficiency. Solar electrical systems often tie right into the electrical power grid, so therefore no batteries are required to store electrical energy for the system. However, a solar hot water system requires sufficient storage capacity to absorb the energy produced by the collectors during the daytime collection hours. Working with many new solar hot water designers, I have noticed that designing in the appropriate volume of solar storage can be overlooked. It’s by no accident that this tendency occurs. A solar collector can be viewed as a boiler — energy is captured and delivered. But unlike designing boilers for peak hot water demand, solar energy is only available for a limited time each day. Good solar hot water design not only includes sizing the solar collector area to carry a percentage of the total hot water or heating load, but also suggests sizing the solar storage vessel to absorb all of the thermal energy the collectors will produce throughout the day. If the solar vessel is not sized large enough to the total collector area and there is no hot water demand, the solar tank could reach the critical high limit temperature and the solar controller will turn off the solar collection cycle. On commercial projects, under-sizing solar storage tanks can lead to chronic problems associated with extreme glycol and tank temperatures, or the poor economic result of having collectors that are not in use as often as they could be, and excessive heat dumping. Because choosing solar storage vessels for commercial hot water systems is a matter of finding large volume bulk storage tanks, this article is to help you become familiar with different types of tanks that can be integrated for solar hot water systems.

Sizing the solar thermal tank

There are no hard and fast rules for sizing storage in solar hot water systems. The general rule of thumb for solar storage sizing is that for every square foot of collector aperture, you will need to store anywhere between 1 to 2½ gallons of water, depending on your location and the application. The strategy is to allow for a reserve, as water, to store the solar collection cycle if very little or no daytime load is present, such as for homes and apartments. In some commercial systems, daytime hot water load constitutes the majority of the total load, and if that is the case, a fully sized solar storage vessel may not be necessary. Buildings that are occupied 365 days per year, such as hospitals and nursing homes, fall into this category. Most commercial buildings, however, have load variations based on occupancy and would require full storage capacity. Hotels, schools, and offices that have weekly or seasonal load variations should size storage capacity for more rather than less.

The goal to meet with a solar hot water system is to raise the temperature in the solar tank so that it is at or above design temperature (i.e. 120 °F for DHW) while having enough storage volume to absorb the remaining daytime collection cycle. Because solar energy cannot be depended upon, and varies widely with cloud cover and seasonal changes, you cannot expect a consistent temperature output from the solar tank day in and day out. Solar radiation, the hot water load, and storage volume all are interrelated and affect the constantly changing temperature at the solar storage vessel. It is a reminder then to understand that solar for domestic hot water is designed as a preheating system.

There is a piping and control strategy for solar storage tanks that would provide a basis for high capacity storage and high output temperature — a solar hot water system that can absorb solar energy during the summer months (or during low- or no-load cycles) and also has a built-in mechanism for driving high temperatures during low solar energy winter months or cloudy days. The concept is to load solar energy into a large vessel by sections, or to prioritize one storage vessel over another auxiliary storage tank(s). This is commonly done with a three-way valve and a solar controller with the capabilities to divert solar energy on temperature rise after priority section or tank is satisfied (i.e. 140 to 180 °F). The practice is not widely carried out but it could be; the design makes better use of solar energy storage and the capability to produce design temperatures across the range of the sun’s day-to-day and seasonal energy spectrum. For buildings with wide load variations and those in the northern climates, this design adapts the storage system to the changing relationship between the daily solar energy available and the hot water load.

The solar storage vessel

There are two basic tank categories for large volume bulk hot water storage vessels — pressurized tanks and atmospheric vessels. Pressurized tanks are commonly installed for residential or light commercial (80 to 120-gallon) solar hot water systems. Commercial solar hot water pressurized tanks can vary anywhere from 200 gallons up to several thousands of gallons. Although pressurized tanks are very common in the hot water industry and are often considered the standard option, atmospheric tanks shouldn’t be ignored for solar commercial hot water storage. There is a distinct advantage that atmospheric vessels have with solar collection systems that gives them an economic edge. The atmospheric tank may have limited practical usage, but can play into enormous gains for long-term system integrity.

Atmospheric vessels are storage reservoirs which contain unpressurized water which is the energy storage and transfer medium. The water in the atmospheric vessel may be pumped into a gravity-drained solar collector system (referred to as “drainback”) or the vessel can contain heat exchange coils installed for “closed loop” glycol solar collector transfer. Any atmospheric tank will need to be well insulated (no heat should be felt on the outside skin of the tank) and sealed tight against escaping evaporative steam. The main advantage of the atmospheric vessels is cost. Typically, dollars spent per each gallon of water stored can be considerably less than that of a same size pressurized vessel. This may not be true for all atmospheric tanks, but for all EPDM lined tanks it is the case. An atmospheric tank lined with EPDM rubber is one low-cost solution to large bulk storage. These can be purchased as square or round tanks and can be assembled on site. Copper heat exchanger coils are normally placed in the tank for energy transfer. The typical lifecycle of an EPDM liner is 12 to 15 years if temperatures are controlled to remain under 170°F. Heat exchange coils may be removed and liners replaced at the end of a liner’s lifecycle.

Another type of atmospheric tank manufactured in Europe has 15 years of technology behind it. It is designed to store hot water temperatures up to 185°F and includes a long 20+ year lifecycle. Packaged and built on site, the water-holding container wall consists of durable glass-fiber-reinforced plastic. These tanks range in size from 350 to 12,000 gallons. Stainless steel flexible heat exchangers are included for thermal energy transfer. This type of atmospheric tank may not be less expensive than a pressurized vessel, but has the advantage of fitting through a mechanical room doorway in individual parts and is built on site. The non-corrosive container wall would also suggest a longer operating lifecycle than that of a steel glass-lined tank.

Pressurized vessels are the most common tanks specified for commercial solar hot water systems. Tanks under 120 gallons are standard for residential and light commercial systems. They can be piped in parallel or in series for larger storage volumes without the costs for ASME-rated tanks above 120 gallons. These small solar storage tanks are constructed of stainless steel or as glass- or enamel-lined steel tanks. Some models are solar ready and include extra ports for heat exchangers or can be purchased with internal single wall heat exchange coils.

For larger pressurized tanks, think about longevity and check with your manufacturer. Large pressurized vessel sizes range from 240 to over 4,000 gallons. Once in place you want them to stay. Small commercial hot water storage tanks of 200 to 400 gallons that operate under heavy usage are usually expected to fail within six years. They are large but designed to move through mechanical room doorways for easy replacement. For solar, the opportunity for a longer lifespan would include a double glass-lined manufactured tank. The process of double glass lining would not only strengthen the integrity of the glass liner itself, but also fill in any gaps the first layer might miss. Also consider tanks that are stone lined. The benefit to a stone-lined tank is not only a longer lifecycle, but that they can be relined to extend the life of the steel and insulation shell. On these 1,000+ gallon hot water storage vessels, manhole ports are included for this service.

Whatever tank you choose to specify, remember the golden rule of solar storage: Solar hot water is for the application of cold water preheating. The preheated solar tank feeds the water heater so the back up doesn’t have to work so hard. This design principle is often not understood. Common mistakes to avoid are: (1) designing solar thermal collectors on a hot water storage tank already being heated to design temperature with a boiler and (2) piping the recirculation hot water return directly into the solar tank. In both cases, the opportunity to preheat cold water is eliminated, and the purpose of solar hot water to save energy is diminished. It just so happens that solar thermal collectors are at their most efficient collection cycle at lower water temperatures, and loading cold tanks increases the system’s storage capacity. Design solar domestic hot water for cold water preheating and you will never go wrong.

Peter Biondo is the technical sales coordinator for Oventrop Corporation. He has been involved in solar hot water and hydronic heating for more than 25 years. His primary work is assisting mechanical engineers and contractors with hydronic heating systems, as well as solar domestic hot water and heating for residential and commercial applications. His solar and hydronic workshops are featured at trade shows throughout the country.