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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.
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.
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.
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.
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