PV Differential

If youíre looking for a 12 VDC solar powered differential thermostat that works in harmony with a photovoltaic power supply the PV Controller may be just what youíre looking for as long as your DC pump does not require more than 20 watts of power. If youíre are powering an AC pump or if you have a DC pump that requires more than 20 Watts of power check out the AC/DC differential controller below or the Basic Differential Controller   as well as some other options on the differential controller page. Some people use only a PV panel to regulate the heat flow from a collector. Bright sunlight on a PV panel will of course produce the power needed to drive a pump faster. Regulating a circulator pump in a closed loop system does work to some extent with a PV panel alone as long as the circulator can handle large voltage swings and the user does not mind discrepancies between solar flux and solar heat.





the efficiency of heat collection can be greatly improved with the addition of a special kind of inexpensive controller. As you know a PV panel connected directly to a DC pump may be used to harvest the sunís heat energy. BUT if youíre only using a PV panel to regulate pump speed the pump may come on when the collector is still cold. Circulators circulate glycol/water solutions through closed loop systems at speeds proportional to the amount of sunlight available to the PV panel, but the temperature of a collector may not always be proportional to the light available to a PV panel.

FOR EXAMPLE: Suppose the collectors are covered with snow in bright sunlight and the PV panel is clear.
What happens? Thatís right, the circulator turns on and warm water from your storage tank is circulated through your ice-cold collectors. Eventually the warmed collector will melt the snow and the collectors may be available to collect heat but there may be no heat to collect if the sun suddenly fades behind a cloud. Collectors are at their coldest in the early morning hours, but if a DC pump is directly connected to a PV power supply stored solar heat may be lost through cold collectors as soon as the morning sun strikes the Photovoltaic panel. This is where the PV Controller comes to the rescue.

12VDC Differential Thermostatic Controllers are used to regulate pumps and automate the process of collecting and storing solar heat. Most DC differential controllers need a battery backup with a charge controller to stabilize the voltage and protect the battery. However a DC controller has been specially developed for closed loop solar hot water systems with or without a battery backup. This innovation is made possible by replacing the mechanical relay with a solid-state relay that may be activated with as little as 4 VDC.

Hybrid (solar panel/solar collector) systems use PV power to drive and regulate DC pumps. Since PV power is proportional to sunlight intensity it seams reasonable to assume that collector temperature is also proportional to sunlight intensity and this is often true.. Unfortunately collectors take awhile to heat up and cool down and photovoltaic panels respond instantly to variations in solar flux.

PV Differential Controllers are designed to work in harmony with PV regulation by turning a pump off and on at the appropriate times and by remaining stable under the erratic voltages supplied by photovoltaic power supplies. Under no load conditions 12VDC panels put out between 18 VDC and 19VDC, but under the load of a pump in dim light the PV supply voltage often drops below 5VDC. When this happens most controllers without a battery backup chatter under the load/no load conditions of a pump turning on and off. This is where the PV Controller comes to the rescue with a time delayed response solid state relay.

Circulators used in closed loop systems are very much different than pumps used in open loop systems. Circulators circulate glycol/water solutions through closed loop systems at speeds proportional to the amount of sunlight available to the PV panel. This is how flow rate and heat collection is regulated. More sunlight means more heat collection. Itís as simple as that. Closed loop systems have a certain simple elegance that make inconsistencies tolerable. Still closed loop systems run more efficiently with a differential thermostat. Suppose itís January 15 and a light snow has covered your collectors and your PV panel and the snow slides off the PV panel and a bright sun suddenly appears. What happens? Thatís right, the circulator turns on and circulates warm water from your storage tanks through your ice cold collectors. Eventually the warmed collector will melt the snow and the collector will be availableÖ Or will it? In most cases the heat gain will more than make up for the heat loss, but what if the sun suddenly ducks behind a cloud for the rest of the day?  Cold evenings with a bright sunset pose another problem. As long as sunlight is available the circulator continues to circulate. Toward the end of a long sunny day the temperature inside heat storage tanks may be higher than collector temperature. It would be a shame to loose heat through a circulator that wonít stop circulating. 

WITHOUT A CONTROLLER: Open loop systems such as Drain Back or Trickle Down or MTD  systems use high head pumps. PV power alone may be too low to pump water to the top of a drainback collector in dim sunlight. If this happens PV power will be wasted on a pump with insufficient power and no heat will be transferred to the heat storage system even though the pump is turning. A battery backup with a max power output at least 2x the pump power requirement is needed to remedy this situation. As long as the DC power demands of the pump do not exceed 30 watts the PV controller can be used.

An alternative to the PV controller is the AC/DC Controller. It has monitors for both collector and storage temperatures. And it can regulate AC or DC pumps.

This an  AC/DC Differential Controller . It responds to the difference in temperature between collector and storage and turns an AC or DC pump on and off to maximize the heat harvest from a solar hot water or solar heating system.  Temperature monitors are used to display the actual temperatures inside the collector and inside the heat storage tank, but the controller will function without them.  This AC/DC controller consumes less than 1 watt of power per hour yet it is capable of regulating 300 Watt pumps.  This is a rugged, dependable, analog Controller.

PV Differential Heat Controller assembled  with probes    FREE SHIPPING                              $85

AC/DC Differential Controller  fully assembled and guaranteed   USA only                                $150

AC/DC Differential Controller  fully assembled and guaranteed international                              $150

Calibrated Temperature  Probe                                                                                                 $10

For shipping to areas other than USA contact:

For simple Differential thermostat Kits with free shipping try this...