Liquid System Thermal Storage

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Two types of water storage for liquid systems are available: pressurized and unpressurized. Other differentiations include the use of an external or internal

Hot water load

Hot water load

From collector

Solar storage tank

Internal coil

To collector ^ controller

To collector controller

From collector

To collector <

Solar storage tank

Tube bundle

To collector

Cold water supply Cold water supply

Immersed coil heat exchanger Tube bundle heat exchanger

FIGuRE 5.16 pressurized storage with internal heat exchanger.

heat exchanger and single or multiple tank configurations. Water may be stored in copper, galvanized metal, or concrete tanks. Whatever storage vessel is selected, however, this should be well insulated and large tanks should be provided with internal access for maintenance. Recommended U value is ~0.16 W/m2-K.

Pressurized systems are open to city mains water supply. Pressurized storage is preferred for small service water heating systems, although in cases like Cyprus, where the water supply is intermittent, it is not suitable. Typical storage size is about 40 to 80 L per square meter of collector area. With pressurized storage, the heat exchanger is always located on the collector side of the tank. Either internal or external heat exchanger configurations can be used. Two principal types of internal heat exchanger exist: an immersed coil and a tube bundle, as shown in Figure 5.16.

Sometimes, because of the required storage volume, more than one tank is used instead of one large one, if such a large-capacity tank is not available. Additional tanks offer, in addition to the extra storage volume, increased heat exchanger surface (when a heat exchanger is used in each tank) and reduced pressure drop in the collection loop. A multiple-tank configuration for pressurized storage is shown in Figure 5.17. It should be noted that the heat exchangers are connected in a reverse return mode to improve flow balance.

An external heat exchanger provides greater flexibility because the tank and the exchanger can be selected independently of other equipment (see Figure 5.18). The disadvantage of this system is the parasitic energy consumption, in the form of electrical energy, that occurs because of the additional pump.

For small systems, an internal heat exchanger-tank arrangement is usually used, which has the advantage of preventing the water side of the heat exchanger from freezing. However, the energy required to maintain the water above freezing is extracted from storage, thus the overall system performance is decreased. With this system, a bypass can be arranged to divert cold fluid around the heat exchanger until it has been heated to an acceptable level of about 25°C (ASHRAE, 2004). When the heat transfer fluid is warmed to this

From collector ►

Hot water load 1

From collector ►

Multiple Thermal Storage
FIGURE 5.17 Multiple-tank storage arrangement with internal heat exchangers.

From collector ►

To collector sensor

Collector pump

Collector heat exchanger

Pump

Hot water load

Hot water load

FiGURE 5.18 pressurized storage system with external heat exchanger.

level, it can enter the heat exchanger without causing freezing or extraction of heat from storage. If necessary, this arrangement can also be used with internal heat exchangers to improve performance.

For systems with sizes greater than about 30 m3, unpressurized storage is usually more cost effective than pressurized. This system, however, can also be employed in small domestic flat-plate collector systems, and in this case, the make-up water is usually supplied from a cold water storage tank located on top of the hot water cylinder.

Unpressurized storage for water and space heating can be combined with the pressurized city water supply. This implies the use of a heat exchanger on the load side of the tank to isolate the high-pressure mains' potable water loop from the low-pressure collector loop. An unpressurized storage system with an external heat exchanger is shown in Figure 5.19. In this configuration, heat is extracted from the top of the solar storage tank and the cooled water is returned to the bottom of the tank so as not to distract stratification. For the same reason, on the load side of the heat exchanger, the water to be heated flows from

Hot water load

Hot water load

FIGuRE 5.19 unpressurized storage system with external heat exchanger.

the bottom of the backup storage tank, where relatively cold water exists, and heated water returns to the top. Where a heat transfer fluid is circulated in the collector loop, the heat exchanger may have a double-wall construction to protect the potable water supply from contamination. A differential temperature controller controls the two pumps on either side of the heat exchanger. When small pumps are used, both may be controlled by the same controller without overloading problems. The external heat exchanger shown in Figure 5.19 provides good system flexibility and freedom in component selection. In some cases, system cost and parasitic power consumption may be reduced by an internal heat exchanger.

Stratification is the collection of hot water to the top of the storage tank and cold water to the bottom. This improves the performance of the tank because hotter water is available for use and colder water is supplied to the collectors, which enables the collector to operate at higher efficiency.

Another category of hot water stores is the so-called solar combistores. These are used mainly in Europe for combined domestic hot water preparation and space heating. More details on these devices are included in Chapter 6, Section 6.3.1.

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Responses

  • greg
    How big are solar liquid heating collection tanks?
    9 years ago

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