Tank configuration and use
A challenge in applying renewable energies is often the mismatch between the time energy is needed and the time energy is available. Thus storage tanks are a necessary part of any hot water system since they couple the timing of the intermittent solar resource with the timing of the hot water load.
For systems that provide heat for domestic hot water, 1 to 2 gal (3.8–7.5 L) of storage water per square foot of collector area are generally adequate. The storage fluid can either be potable water or non-potable water if a load side heat exchanger is used. For small systems, storage is most often in the form of glass-lined steel tanks. Solar heated water may be stored in a “one-tank” system, or it may be stored in a separate tank that feeds into the tank of a conventional gas or electric water heater (a “two-tank” system). Whether one or two tanks are used, solar energy heats the water before use. On sunny days, a typical solar system can raise water to 140 °F (60 °C).
Bigger commercial solar hot water systems are basically the same as those used for homes, except that the thermal storage tank, heat exchanger, and piping are larger. The storage tanks in these applications are commonly steel tanks with an enameled interior coating. The sizes of these components are proportional to the size of the collector array. Most systems include a backup energy source such as an electric heating element or are connected to a gas or fuel fired central heating system that will heat the water in the tank if it falls below a minimum temperature setting, enabling the system to work year-round in all climates.
If the solar hot water system provides for some of the building heat, a larger storage tank may be advisable. Figure 3.28 shows a breakdown of common storage types.
Most large systems have stratified storage tanks where cooler temperatures are at the tank bottom and the hotter temperatures are at the top. The cooler fluid is drawn off the tank bottom and is sent to the collector system for heating. It may go directly to the collector or be used to cool the heat transfer fluid that is then sent to the collectors. Using this cooler water, increases the collector efficiency. The heat transfer fluid is held in the collector until it reaches the desired hot temperature. This is accomplished by stopping the fluid flow or by slowing the flow in the collector until the desired temperature is reached.
Thermal losses from the storage tank are a significant part of the heat balance of the solar thermal system. The losses are proportional to the surface area of the storage tank. Because the volume of a solid body (e.g., cylindrical storage tanks) increases faster than its surface, larger storage tanks have a lower heat loss per volume than smaller tanks. Combining multiple storage tanks into one large storage tank is thus beneficial regarding reducing storage heat losses. All storage tanks need to be insulated to reduce the amount of heat lost from the system. It is good practice to limit the heat losses to 10% of that in the storage tank over 24 hrs. An insulation value of R-16 is the minimum insulation required. If the tank is placed outdoors the insulation should have a weather proof cover.
The storage tank temperature must satisfy the required service temperature and quantity. For DHW, this would be 140 °F (60 °C). For other heating needs the temperature could be hotter. A hotter temperature than the service requirements in the storage tank allows for greater storage of heat, but reduces the collector efficiency. This consideration for the storage tank normally results in a design that provides a stratified water condition in the tank with the hot water on the top and the cold water at the bottom. Stratification of the water is naturally achieved and maintained if mixing of the water is minimized since the cooler water has a higher density and tends to stay at the tank bottom. For good stratification, the tank should be as tall as possible, which means a high, narrow tank is best. Such a tank could be divided into several shorter tanks plumbed in series where the outlet at the top of the first is connected to the inlet near the bottom of the second and later tanks similarly piped. Up to four tanks are often installed in this manner for large systems. If one large tank is used, there may be a pipe connection in the middle section that allows for the entry of water at a temperature warmer than the cold water at the bottom, but not as hot as that in the upper part of the tank. Using this technique minimizes the mixing of the stored water and helps keep the water column stratified.
For larger storage tanks it is common not to store drinking water in the whole storage volume, but to use an internal or external heat exchanger through which drinking water flows for heating (see Figure 3.29). This avoids Legionella growth at mid temperatures, eliminating the need to heat to higher temperatures, more than 140 °F (60 °C).
The most commonly used storage tank systems are used for short-term storage, and are designed to store surplus solar energy for 1–2 days (diurnal storages). This limits the possible solar fraction* (SF) to 10 – 20% of the total heat demand (space heating + DHW) supplied by the solar system, but provides the lowest system investment cost.
Long-term storage tank systems can compensate for seasonal fluctuations in solar irradiation between winter and summer, which can include solar fractions in the range between 40 and 70%. The negative effects are higher system investment costs and higher heat losses of the system. Between short and long-term storages, weekly or medium-term storages have been realized in Austria.