Controls
There should be an automatic control system that senses system performance and properly transfers collected heat to the intended users. Since the amount of heat being collected can vary as does the demand for heat by the users, the control system must tract both of these variables and operate the systems equipment to achieve the maximum performance. The control system accomplishes this by taking information from sensors and, through the controllers, operates pumps, adjusts valves, and activates the auxiliary heating system.
To correctly monitor the solar hot water system, is important to understand system operation and to maintain good efficiencies. The collection of operational data will assist in the tracking of system performance and aid in scheduling system maintenance. This monitored information shall be collected every day of the year on 15-minute interval. All monitor systems should be centrally monitored for consistency; operating personnel must maintain a familiarity with the hardware and software, and the system must be properly managed through trend tracking and effective dispatching of service personnel. The minimum monitoring points are:
- tank temperature
- solar array circulating pump start/stop
- solar array circulating pump status
- solar controller alarm
- solar array inlet temperature
- solar array discharge temperature
- solar array discharge temperature alarm high limit.
The temperature sensors that measure the collector discharge temperature should be monitor the absorber plate temperature near the collector outlet. The storage tank temperature should be at the tank bottom to obtain the coldest temperature. These sensors send this collected information to a controller called a differential temperature thermostat, which then compares it with the adjustable setpoints – usually the high and low values. When the high value is reached (typically 12 °F to 15 °F (-11 to -9.44 °C) an action to withdraw the heat from the collector is initiated; for example starting a pump. As the pump runs the temperature differential drops and after some period of time the low setpoint is reached (typically 4 °F [-15.56 °C]). At this temperature difference, the pump is stopped. The pump is restarted when the high limit of the thermostat is again reached.
When a system has freeze and/or overheating protection, the controller takes the appropriate actions. For example, when freeze setpoint is reached at the collector, the pump could be activated to pump warm water from the storage tank into the collector. The freeze protection setpoint should be set at 40 °F (4 °C) since heat can radiate from the collector to the night sky creating collector freezing conditions above 32 °F (0 °C) outdoor temperatures. The freeze protection sensor must be placed on the collector so that it will sense the coldest water in the collector. This location may be the collector intake or return manifold, the back of the absorber plate near the bottom or center of the collector. The collector center is identified to monitor the irradiation leaving the collector at night. More than one sensor can be used for this function.
The energy produced by solar energy shall be determined by a Btu meter that will measure the domestic hot water flow from the storage tank, the incoming cold make-up water temperature, and the hot water temperature leaving the domestic hot water storage tank. These values should be continually evaluated and compared to previous values to assure proper performance is maintained.
Overheating controls would be initiated when the collector temperature reaches a temperature in the range of 200 to 250 °F (93 to 121 °C) depending on the system. The sensor monitoring this condition would be placed on the back of an absorber plate in the collector. When the collector reaches the high temperature setpoint due to a pump failure or some other event, the controller can take actions to relieve the heat and protect the system. See section 2.4.2 for more information.
The control components are the same that would be used on a commercial hot water heating system.
A typical sequence of operation for the operation of the solar collector circulation pump that sends water to the storage tank is:
- System run command:
If solar array temperature is above the solar tank temperature by 12 °F (-11 °C), energize solar collector circulation pump P-1. Pump shall be energized during daylight hours only. On command to start, pump must remain energized for an adjustable period of time (initial setting of 20 minutes). - System stop command (high temperature):
If solar collector circulation pump P-1 is energized and if solar storage tank temperature rises above 185 °F (85 °C) (adjustable), activate a high temperature alarm. If solar storage tank temperature rises above 195 °F (91 °C) (adjustable), de-energize solar circulation pump P-1. - System stop command (low temperature):
If solar collector circulation pump P-1 is energized and solar array discharge temperature falls below the solar tank temperature for a period of 10 minutes (adjustable), solar collector circulation pump shall deenergize. - Thermal shock prevention:
If solar circulation pump P-1 is de-energized during daylight hours and if solar array temperature is below 180 °F (82 °C) (adjustable), solar circulation pump shall be enabled.
If solar circulation pump P-1 is de-energized and if solar array temperature is 180 °F (82 °C) or above (adjustable), solar circulation pump P-1 shall be disabled for an adjustable time period (8 hr initial setting).
The sequence of control assumes a constant speed pump circulating the heat transfer fluid through the collectors. If a variable speed pump is used, then the pump speed could be altered instead of stopping and starting the pump. Of course the pump operation would be stopped when night begins and when excessive temperatures are reached in the collector. Another option would have a threeway valve that would allow the heat transfer fluid to circulate through the collector again if it is not hot enough to be placed in the storage tank.