Solar Controllers

The action of the controller is vital in bringing together the various subsystems that we have looked at in an efficient and safe manner to maximize the yield of the system. In the diagram below the fundamental points where the controller interfaces with the collector, cylinder and solar loop pump are shown.


Solar controllers are sometimes known as differential controllers. When the panel becomes warmer than the bottom of the cylinder by a certain number of degrees (normally 4°C to 8°C) it switches the Solar pump on. The panel then begins to cool, when the panel temperature has dropped to about 2°C to 4°C warmer than the cylinder the pump is turned off allowing the panel temperature to rise again. The Controller then repeats the cycle.

However, this method is not without its problems. The temperature sensor is normally in contact with the copper manifold, and it can take up to 30 seconds to correctly resister that the water (or antifreeze mix) temperature. This can lead to the panel not performing at it's optimum efficiency.

This problem is overcome to some extent if the panel flow is restricted. This means that under full sunlight conditions the energy being collected by the panel raises the temperature of the water flowing through the panel by between 6°C and 15°C. This means that the ON/OFF controller, stays ON throughout the day, preventing temperature hunting. To achieve this, many manufacturers suggest setting a flow rate of between 0.5 l/min to 1 l/min for each M² of panel.

A better way of course is to use a modulating controller, which automatically adjusts the flow rate to keep a constant temperature differential between the collector and the cylinder. This is explored in more detail later.



The reason for the hysterisis in switching levels (the higher switch on differential and the lower switch off differential) is to maximize the solar yield without drawing heat unnecessarily from the bottom of the cylinder and to avoid running the pump when there is no heat to be collected. The simplified temperature graph below illustrates how the differential switch occurs in practice. As the collector heats up its temperature rises above that at the bottom of the cylinder by more than the set differential, often about six degrees.



This temperature difference is chosen to so that the solar system will come on only when there is enough energy arriving at the collector to ensure that it stays on. Too low a differential means that the collector will quickly cool, allowing the temperature of the collector to drop below the lower differential set-value and causing the pump to switch off. This on-off cycle generally draws energy unnecessarily from the bottom of the cylinder and allows it dissipate in the solar loop pipework. As can be seen from the graph cooling of the collector tends to occur initially as the cooler water from the solar loop pipework enters the collector. As long as a time delay in the collector holds the pump on for a few minutes, the collector temperature will recover and the system will stabilize without any unwanted hunting on and off. Setting the differential value too high will mean that there will be a longer delay in bringing on the pump at a time when there is energy to be collected, this lowers the collector efficiency. The lower switch-off differential is set at about two degrees typically, as below this the pump will be running while no net energy is being stored in the cylinder. Electrical costs will rise and the marginal heat rise from the collector is dissipated in heat losses in the return pipework. The graph below illustrates the system switching off as the cylinder temperature rises to within two degrees of the collector temperature. The system would also turn off if heavy cloud obscured the collector causing it temperature to be cooled below the switch-off set-point by the cooler water from the cylinder.

Sorel TDC3

Established in 1991, Sorel GmbH is a highly respected manufacturer of solar thermal control systems. The Sorel product range provides a suitable control solution for all requirements. Solar Thermal and domestic water heating systems including Swimming pools. Sorel products are manufactured under ISO 9001 : 2000 with TUV certification. A very straight forward graphical interface (with an on-line help function) makes it very easy to commission as well as letting the end user easily access energy usage statistics. The Sorel heating controllers are designed to save energy by optimizing and continuously tuning heating systems by adjusting operating parameters based on ambient weather conditions. The same philosophy has been extended to the Solar controllers using intelligent algorithms to minimize pump electricity usage while simultaneously maximizing the efficiency of the solar panel. Allied Solar has been awarded exclusive rights to market the Sorel line of controllers in Ireland (both North and South).

Weather patterns are far more variable in Ireland and the UK than in central Europe where most controllers are designed. Early solar controllers (and cheaper controllers now) provide a differential function described above along with a facility to turn the pump off once a certain maximum cylinder temperature was reached.

Typically this temperature was set to 60-65°C, to avoid problems with the excessive formation of scale in harder water areas at temperatures above 65°C. In areas without scale problems, the temperature could be allowed to rise further to perhaps 80°C. Safety wise, it is very important that a thermostatic mixing (Anti-Scald) valve is fitted. At higher temperatures a greater amount of heat is stored in a given cylinder, which is blended down with cold water to the desired temperature.

Variable Speed Pumps

A central heating pump contains a simple induction motor which normally works at a constant speed directly related to the mains frequency . Although the user can select one of three internal coils to select different pumping speeds, the solar controller generally cannot switch coils.

To change pumping speed a controller generally uses a very fast electronic switch (MOSFET) to pulse the pump with power so that it turns more slowly.

The mechanical power needed to pump water / antifreeze mix is proportional to the cube of the fluid flow. Thus, for instance, reducing the flow from 100% to 80% of the nominal value would halve the electrical power required.

Secondly it matches the pump speed to the available solar energy. This keeps a steady “delta-T” and avoids the inefficiencies the inevitably result because of the delay of the sensor in reading actual panel temperature. (i.e. panel over-heats before pump is turned on and over-cools before solar pump is turned off).

Considerable electrical energy savings are especially to be archived by not reducing the flow rate on the front of the pump, and by using a controller with this functionality.

Normally the cost difference between a controller with on-off functionality and one with variable speed capability is very low, costing in the region of €10 to €30 extra.

PV-based control systems with DC pumps

These can be subdivided into two groups, the first where a single solar photovoltaic PV panel is directly wired to a matched pump and the second where the PV supply is wired through a controller before being connected to the pump. In our climate the second method is to be preferred, as there will be occasions when the solar irradiance is bright enough in cold weather to power the pump without there being additional heat to be collected from the thermal collector. This could draw heat from the bottom of the cylinder and dissipate it in the collector. the second method retains the differential control and prevents heat loss. The controller may also draw its power from the PV panel and thus there are no parasitic losses from the solar system, allowing it also to be used off-grid and, more generally, without the need for a mains connection. As these solar systems usually use a low-flow approach the actual power of the DC pump may only be of the order of 5-20W and therefore a fairly small PV panel will suffice to power it.

Temperature Sensors

Temperature sensors are required at a minimum up at the outflow from the collector and towards the bottom of the hot water cylinder (usually between the coils), with a third temperature sensor sometimes installed at the top of the cylinder. There are two main types used, thermocouples (NTC & PTC) and resistance temperature detectors (RTDs). Because of higher accuracy and repeatability, RTDs have generally replace thermocouples in solar applications, some chinese controllers are still using thermocouples to sense temperatures at the cylinder. Normally 12V (but always a low voltage) is placed across the RTD or Thermocouple, and the current is measured to give the resistance reading and hence the temperature. RTDs are generally made of platinum, and come in three (main) flavours.

• PT100, 100 Ωs at 0°C
• PT500, 500 Ωs at 0°C
• PT1000, 1000 Ωs at 0°C

The PT1000 increases in resistance by 3.85 Ω per °C
At 30 °C, the resistance would be 1115.5 Ωs

Resistance of PT1000 extension leads

A very high 2Ω resistance on an extending lead (which is HUGE) would lead to a 0.5°C error in reading temperature using a PT1000. Earlier controllers also used PT100, which exhibit a small increase in resistance (0.385 Ω / °C). A 2Ω resistance here would lead to a 5.2 °C error, which is much more significant. As a result, earlier texts encourage a 0.5mm² leads up to 50meters, and 0.75mm² leads over this distance. If a PT1000 is used in the solar circuit, smaller diameter leads can be used, and indeed "bell wire" at €4.00 for 100 meters makes a very good low cost solution.

Another advantage in using a PT1000 is because the resistance is 10 times higher than the PT100, 10 times less current is used to measure the resistance, which in turn means lower parasitic electrical losses.

Controller Freeze Protection

There are two ways to protect against solar panel pipes freezing. The first way is to use the controller function. However this can only be used in some circumstances when using heat pipe solar collectors. The Sorel TDC3 controller has two anti-frost settings, below which the first turns the pump on for 1 minute every hour. (e.g. at 3C), if the manifold temperature reaches a second setting (e.g. 1C) then the controller runs the pump continuously until the temperature recovers. This is adequate most of the time, however in rural areas where freezing conditions may coincide with a power outage, this system can leave a solar panel exposed to freezing. It may be deemed a small risk and the consequences are not catastrophic if pipe work to the panel were to fail. Most likely the leak would be outside the building and because the system is closed only a couple of litres would leak.

The other alternative is to fill the system with an Anti-freeze mix, in any case this is necessary if a flat plate collector or primary fluid vacuum tube panel is used. The Anti-Frost function on the controller can also be used to give a "belt and braces" solution, if the anti-freeze mix gave protection to -10°C, the controller could be set to circulate the mix at -3 °C which is still very rare in Ireland, but offers a much higher protection, particularily if the anti-freze mix has degraded (with time & overheat) and can no longer offer the original protection.

Link here to discussion on anti-freeze percentages.

Heat metering

At a basic level a controller may offer the ability to provide an estimate of the heat in kWh supplied by the solar collectors to the cylinder by multiplying the difference in temperature between the collector and return from the cylinder by the approximate flowrate (taken from the pump speed, set-up at commissioning). The estimation in flow rates will result in estimates of heat gains that are probably about +/- 20%. A better estimate will be achieved if the controller can take an additional digital input from a flow meter inserted in the solar loop.

Data Logging

Several controllers offer either a self-contained ability to record the system variables over time or can link to an accessory datalogger or even to a BMS Building Management System. Although generally deemed an unnecessary additional expense in a domestic system, it may become essential if ROC payments for renewable thermal generation become available. On top of this, it is a valuable check that the system is working with something approaching its design performance. Since solar systems always operate with an auxiliary back-up that may be set to automatically compensate for solar shortcomings, then it can be difficult to determine actual solar performance, particularly given the variability of solar irradiance.

If heat-metering becomes established for the payment of Thermal ROCs in the UK, then probably only a meter from a recognised list will be permissible to verify the number of units generated. This is no doubt make its way to Ireland.