Active Systems a

Active Systems require a pump to circulate the water from the solar storage tank through the collectors. There are two general categories of active solar water heating system – open loop and closed loop systems.

Open Loop System

In these types of systems the water stored in the solar storage tank is circulated directly through the solar collectors where it is heated and returned to the storage tank. Open loop systems are very basic and were the first to appear on the market many years ago. The concept is very simple. A pump controller (called a differential controller) will turn the pump on when the collectors are hotter than the stored water and shut it off when they reach the same temperature. The benefits to this system are cost and ease of installation, but the drawbacks often outweigh these benefits. First, there is no totally reliable form of freeze protection other than manually draining the collectors. Therefore they should only be considered in areas that do not freeze, ever. A second problem is that there is no high limit protection. By this I mean that without daily consumption of hot water, the water in the storage tank and solar collector can boil. If this occurs, relief valves (when operating properly) will release. This can be messy and a waste of energy. Finally, in open loop systems, the solar collectors are susceptible to clogging from mineral deposits in areas of problem water. Figure 1 depicts a typical open loop system.

Figure 1. The open loop system.

Closed Loop Systems

Unlike open loop systems, closed loop systems do not directly heat the water in the storage tank. By this I mean the water in the tank does not circulate directly through the solar collectors. There are two types of closed loop systems – the glycol indirect system and the drainback system. In this type of system a separate fluid is circulated through the collectors and a heat exchanger is used to transfer this heat to the water in the storage tank. This heat exchanger can either be built into the storage tank, or separate from it. If separate an additional pump is required to circulate water from the storage tank through the exchanger. The heat exchanger fluid is typically propylene glycol. Additional items such as an expansion tank, pressure gages, and fill valves are also required.

Glycol Indirect System
Like the open loop system, the glycol closed loop system has been around a very long time. It also alleviates two of the three problems encountered in an open loop system. By using glycol as the heat transfer fluid the chance of freeze damage is eliminated. Additionally it eliminates the problem of fouling the collectors in areas that encounter hard water. There still remains the problem of high limit protection and the potential of failure of the additional components added (expansion tank, pressure gages, etc). Figure 2 depicts a typical indirect glycol closed loop system.

Figure 2. Indirect Glycol Closed Loop System.

Drainback System

The drainback system was first introduced in a large scale back in the 1980’s. They have been proven very reliable and are more efficient than the indirect glycol system. This is a non-pressurized closed loop system using water as the heat transfer fluid. A small drainback reservoir is installed in the collector loop. When this system is filled with water it is only filled to the top of reservoir. Since it is located below the collectors, they remain dry when the pump is not circulating. When the collectors are hotter than the water in storage, the pump circulates the water in the reservoir through the collectors where it is heated. The heat from this water is then transferred to the solar storage tank through a heat exchanger located either in the storage tank or drainback reservoir. When the collectors approach the same temperature as the water in the storage tank or this water has reached a preset temperature, the pump shuts off and all the water drains back the reservoir. The drainback system eliminates all the problems inherent in the other types of systems. First, freeze protection is based on gravity. Second, the pump is shut off when the storage tank reaches its high limit setting and all water is drained from the collectors. And finally, the problems encountered in the collectors in hard water areas are eliminated. The drainback system also uses fewer components than any of the other types of active system, thus the potential of component failure is drastically reduced. Figure 3 depicts a typical drainback system.

Figure 3. Drainback Closed Loop System.