Solar Hot Water
This section is intended to be a general
overview of typical methods of heating household water by means of the
sun. Synergy Energy Solutions has looked at all the various options
and designed a system that is most applicable for your climate conditions
in the Sunbelt (southern USA) where freezing is limited to less than 2-3
days per year. These systems are direct pumped systems that
are either differenetially controlled or photo-voltaic (PV) operated
systems. For northern climates, please contact us
to discuss 'Indirect' systems.
Direct Pumped Systems
The direct pumped system, illustrated in Figure 1, 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 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 flow through the collector.
The collector should also allow for manual draining by closing the isolation valves (located 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.
Figure 1. Typical direct pumped system
The system shown in Figure 2 , differs from other direct pumped systems in that the energy to power the pump is provided by a photovoltaic (PV) panel. The PV panel converts sunlight into electricity, which in turn drives the direct current (dc) pump. In this way, water flows through the collector only when the sun is shining.
The dc pump and PV panel must be suitably matched to ensure proper performance. The pump starts when there is sufficient solar radiation available to heat the solar collector. It shuts off later in the day when the available solar energy diminishes. As in the previous systems, a thermally operated valve provides freeze protection.
Common appliance timers also may control solar system operation. These timers must incorporate battery backup in the event of power failures.
The timer is set to operate during a period of the day when solar radiation is available to heat the potable water. In order to avoid loss of energy from the tank during overcast days, the collector feed and return lines are both connected at the bottom of the storage tank with a special valve. During normal operation, natural stratification allows the warmer water to rise to the top of the tank.
Figure 2. Direct system with photovoltaic-powered pump
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