Active systems
Solar water heating systems that rely on electric pumps to circulate fluid through the collector are called “active systems.” Active systems are generally categorized into two types: direct and indirect, which simply means that water in the storage tank is either directly filled using the hot water flowing from the solar collectors (one loop) or indirectly using two water circulating loops separated by a heat exchanger. The latter type is normally used in locations where outdoor winter temperatures below freezing may occur. These systems use an anti-freeze solution such as a water glycol mixture as a heat transfer medium that circulates through the collectors to avoid freezing.
Direct circulating active system (no anti-freeze). These systems use pumps to transfer the sun’s energy directly to potable water by circulating this water through the collector tubing and storage tank; no anti-freeze solution or heat exchanger is used. The pumps circulate water through the collectors, into the building, and back again. They work well in climates where it rarely freezes.
A direct active system (Figure 3.5) has one or more solar energy collectors installed and a nearby storage tank. The system uses a differential controller that senses temperature differences between water leaving the solar collector and the coldest water in the storage tank. When the water in the collector is about 15 to 20 °F (-9 to -7 °C) warmer than the water in the tank, the pump is turned on by the controller. When the temperature difference drops to about 3 to 5 °F (-16 to -15 °C), the pump is turned off, so the stored water always gains heat from the collector when the pump operates. A flush-type freeze protection valve installed near the collector provides freeze protection. Whenever temperatures approach freezing, the valve opens to let warm water flow through the collector. The collector should also allow for manual draining by closing the isolation valves (located at a height above the storage tank) and opening the drain valves. Automatic recirculation is another means of freeze protection. When the water in the collector reaches a temperature near freezing, the controller turns the pump on for a few minutes to warm the collector with water from the tank.
Another type of direct active solar water heating system is called “a drainback-system” (Figure 3.6), which is also designed for cold climates. This type of system typically uses regular water as a heat transfer fluid, and is designed to allow all of the water in the solar collector to “drain back” to a holding tank in a heated portion of a building. When no sunlight is available for heating, the solar pump turns off and the water flows into the drainback tank by means of gravity. Since these systems use water, they can be designed with or without a heat exchanger.
Indirect circulating active system (anti-freeze used). This system operates similar to the direct active system except that there are two circulating heat transfer fluid loops. In the first (solar primary) loop non-freezing, heat-transfer fluid such as a water-glycol mixture circulates through the collector field. A heat exchanger transfers the heat from the water-glycol mixture into the potable water. The heat exchanger can either be directly integrated into a storage (internal heat exchange) or the storage can be connected to a second loop (solar secondary loop) via an external plate heat exchanger (Figure 3.7). These systems are popular in climates that are prone to freezing temperatures.
If the solar collector is extremely well insulated and is not prone to freezing like an evacuated tube collector, water can be the heat transfer fluid. By using water the heated water from the collector can be sent directly to the storage tank; no heat exchanger is needed. Figure 3.8 shows the portion of this system before the storage tank (and, if used, before the heat exchanger).
Indirect circulating solar water heating systems used on individual buildings or groups of buildings are consistent with the US Army’s needs. They can easily be applied to central heating systems for domestic hot water and building heating. Since the purpose of this design guide is to focus on systems that can serve multiple buildings, these systems will be emphasized in further sections of this course.