Direct Open Loop

Direct Open Loop System

Differential Controller Operated System


The direct pumped system has one or more solar energy collectors installed on the roof and a storage tank somewhere below, usually in a garage or utility room. A pump circulates the water from the tank to the collectors and back again. This is called a direct (open loop) system because the sun’s heat is transferred directly to the potable water circulating through the collector tubing and storage tank; no anti-freeze solution or heat exchanger is involved.

This system has 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 15-20° F warmer than the water in the tank, the pump is turned on by the controller. When the temperature difference drops to about 3-5° F, the pump is turned off. In this way, the 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 for manual draining by closing the isolation valves (located above the storage tank) and opening the drain valves. Another method of freeze protection is achieved by water recirculation. When the temperature approaches freezing, the pump activates to circulate warm water through the collectors.

Direct Open Loop

Direct Open Loop PV

Direct Open Loop PV System

Photovoltaic Operated System

The direct pumped system has one or more solar energy collectors installed on the roof and a storage tank somewhere below, usually in a garage or utility room. A pump circulates the water from the tank up to the collector and back again. This is called a direct (or open loop) system because the sun’s heat is transferred directly to the potable water circulating through the collector tubing and storage tank; no anti-freeze solution or heat exchanger is involved.

This system has a photovoltaic cell that senses when there is enough solar insolation available to heat the home’s water. The PV cell powers a DC pump which circulates the water from the storage tank to the collector.

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 also allows for manual draining by closing the isolation valves (located above the storage tank) and opening the drain valves.

Direct Open Loop PV

Closed Loop

Closed Loop System

A closed loop system design is common in northern climates, where freezing weather occurs more frequently. An FDA approved heat exchange solution circulates through the collector, and a heat exchanger transfers the heat from the solution to the tank water. Generally, if the heat exchanger is installed in the storage tank, it should be in the lower half of the tank.

In the system shown here, a heat transfer solution is pumped through a closed loop which includes the collector, connecting piping, an expansion tank and a heat exchanger. Heat is harvested in the collector and transferred to the potable water through the heat exchanger in the lower half of the storage tank. An alternative of this design is to wrap the heat exchanger around the tank, which keeps it from contact with potable water.

The brain of the system is a differential controller. In conjunction with collector and tank temperature sensors, the controller determines when the pump should be activated to direct the heat transfer fluid through the collector.

We recommend using a flood grade glycol solution which is a mixture of distilled water and anti-freeze. This type of fluid freezes only at extremely low temperatures so the system is protected from damage cause by severe cold.

Closed Loop

Drainback

Drainback System

A fail-safe method of ensuring that collectors and collector loop piping never freeze is to remove all water from the collectors and piping when the system is not collecting heat. This is a major feature of the drain-back system. Freeze protection is provided when the system is in the drain mode. Water in the collectors and exposed piping drains into the insulated drain-back reservoir tank each time the pump shuts off. A slight tilt of the collectors is required in order to allow complete drainage. A sight glass attached to the drain-back reservoir tank shows when the reservoir tank is full and the collector has been drained.

In this particular system, distilled water is recommended to be used as the collector loop fluid-transfer solution. Using distilled water increases the heat transfer characteristics and prevents possible mineral buildup of the transfer solution.

During the day, the pump is activated by a differential controller. Water is pumped from the reservoir to the collectors, allowing heat to be collected. It is then circulated through the heat exchanger at the bottom of the solar tank. The heat exchanger transfers heat from the collector loop fluid to the potable water in the solar tank.

Drainback