Solar Hot Water

Some typical solar hot water loop configurations are shown on the Solar and Auxiliary Heating Loops help page. This page provides more information on how to configure and control solar hot water systems.

Solar Heating System Control

To provide control over the collector loop, a differential thermostat option is incorporated within the solar loop dialog. The differential thermostat compares the temperature in the water heater to the temperature in a collector so that the loop pump is only turned on when there is a useful heat gain.

 

 

There is an option on the solar collector dialog to allow you to select which collector will act as the sensor for the thermostat. The outlet temperature from the water heater is used as the location for the other thermostat sensor.

 

Note: If the two temperature differences on the solar collector dialog are too close, it is possible for the system to turn on and off rapidly without much useful heat gain. This can also occur if the flow rate through the collector is too high. Without flow the fluid in the collector heats up more quickly; when high flow is turned on, all of the hot fluid is removed and the temperature drops, forcing the system off again.

Freeze Prevention

In climates with a cold season, the solar heating system must be designed to avoid the risk of fluid freezing in the solar collector or exposed pipes and causing damage. There are several strategies that can minimise the risk.

 

  1. Seasonal schedule. The simplest strategy is to not use the system during the cold season. However, this is problematic because it requires the collector to be manually drained of all fluid. The benefits of the solar heating system are also lost during this time. This can be simulated using an appropriate pump flow schedule for the collector system.
  2. Antifreeze. The freezing point of the liquid is decreased by adding antifreeze (glycol) to the water or using a different heat transfer liquid with a lower freezing point. This can be achieved by selecting a glycol fluid for the loop fluid type on the solar loop dialog, this is the default setting.
  3. Drain-back system. This strategy automatically empties the collector when the pump is not running. This scenario is modelled by default in EnergPylus, although the extra pump energy required to start the system is not taken into account.
  4. Re-circulation system. This strategy automatically re-circulates warm liquid from the storage tank back through the collector to maintain the system above the freezing point. There are system losses using this method. This can be simulated by using the Low temperature protection setting on the solar loop dialog to force the system to turn on when the outdoor air temperature or collector outlet temperature falls below a specified minimum.

 

Additional Controls

In addition to freeze prevention, it is also necessary to prevent the system from becoming too hot. This is usually a safety issue for the water heater. For this case it is important to have a high temperature cut-off to stop the pump before damaging the water heater. This is accomplished using the High temperature protection setting on the solar loop dialog (see above).