Solar controller

In the context of solar hot water systems, a solar controller is a device which ensures that the system works correctly.

Components

The simplest solar controller is a comparator with two temperature inputs and an output to control the pump. When the temperature of the solar panel's output is greater than the cold input to the consumer (heating system or thermal store) the pump is switched on to circulate the working fluid through the panel. Electronically this can be, and frequently has been, done with little more than a 741 op-amp IC and a relay. With only a handful more components, some hysteresis may be added to avoid short-cycling the pump (this would be wasteful, as it increases heat losses into cold pipework and may also be wasteful in a drainback solar panel).

A more typical approach for modern systems is to use the ubiquitous microprocessor. In a commercial product this is also likely to offer a simple LCD display and a minimal user interface of a few pushbuttons. The same two inputs have temperature sensors: one for the hottest part of the solar panel and the other for the coolest part of the water store, which is at the bottom. The other two connections are power in and switched power out to the pump. The power can come in from a mains electric supply, in the case of low carbon solar technology. In the event of the latest innovation, a zero carbon controller, power is sourced wholly from a photovoltaic (PV) module which also acts as a power supply.

Other components beyond the basics mentioned above include extra temperature sensors for higher up on the water store, because it gets hotter the higher you go up it, and an LCD or other display. This display can tell the user things such as whether the pump is switched on, what temperature the water at the top of the store is at. This is the temperature of the water which will come out of the taps and therefore it is a useful temperature to know about.

Function

Its main function is to control when the solar panel's pump is switched on or off. The pump is usually switched off when the solar panel is colder than the bottom of the water store and it is switched on when the panel is hotter. Switching the pump on allows the heat in the panel to be transferred to the store. Switching it off prevents the export of bought-in energy. Every few seconds, the temperatures in the panel and the store are measured and compared in order to allow this on or off decision to happen.

In addition, certain fine tuning can take place, such as allowing an overrun time to ensure that heat energy is not left lying about underlivered in interconnecting pipes when the pump is turned from on to off. Another fine tuning is that of the on differential (which may be say 4-15 degrees Celsius) and the off differential which is usually a few degrees lower. The wider the difference between these differentials, the fewer pump on-off cycles will take place. These factors are usually set by the solar installer in relation to the particular installation. The controller may also control certain safety features such as by permitting heat export when the hot water exceed a preset temperature such as 65^oC. This is a process of allowing the solar panel to export excess heat if it is not being used when the panel is cool, when light levels fall towards the end of a sunny day and is used in a range of solar thermal technnologies such as solartwin.

Photovoltaic powered solar controller

A photovoltaic (PV) powered solar controller uses solar electricity produced on-site to run the pump that delivers the solar-heated transfer fluid to the hot water store.

One claimed advantage of PV power is that it reduces the overall carbon emissions associated with operating the system since it avoids the need to supply this energy from fossil sources.^{[citation needed]} However, the energy required to operate the system is very small in comparison to the energy produced by the system and the carbon emissions reduction of adding PV power fractional.^{[citation needed]}

The most practical benefit of a PV powered controller is the resultant simplicity of the overall system. Rather than using complex algorithms based on store and panel temperatures, the pump is driven directly by the PV panel: when the sun shines, the pump runs. In practice this is as efficient a practical control algorithm as most others achieve and has obvious advantages for reduced system complexity.^{[original research?]}

A disadvantage to the PV powered approach is that the pump stops immediately after the sun is occluded. With vacuum tube and heat pipe solar panels, these can have an appreciable amount of energy stored in each tube at the moment the sun goes in. To avoid overheating the tubes it is necessary to either pump their circuits for a short time after the sun, or else to provide a large reservoir of fluid in the header tank above the panel. Neither of these options is really compatible with the simple direct-PV pump approach and so such systems are limited to using the less efficient flat panel collectors.^{[citation needed]}

A PV powered controller may contain a small electricity store to allow the controller to remain powered and display temperatures at night when there is no sunlight. This electricity store is usually in the form of supercapacitors, since these have a much longer life than batteries.^{[citation needed]}

The benefits of a PV powered solar controller comes at a cost in reduced system performance in the range of 1-10%.^[1] This is due to heat losses at times when the panel may be hotter than the water store but there is insufficient sunlight to power the pump. This happens mainly on hot days when hot water is likely to be in excess so the potential reduction is less significant than it would be at times when the store was cooler.^{[original research?]}

Currently (2007) PV powered controllers are a minor sub-technology within a minor technology; it is however possible that the global solar thermal industry will start to adopt PV powered controllers more widely in the future.^{[original research?]} This may happen if the terms of reference of solar thermal start to turn away from maximising component efficiency, which is usually regarded as efficiency per square metre of panel, and moves towards system sustainability.^{[original research?]} System sustainability may be assessed in a variety of terms, for example, operational carbon input/output ratio. This ratio is zero for a range of zero carbon solar technologies such as PV pumped solar, thermosiphon solar and integrated collector and store solar systems.^{[original research?]}

References

^ Martin C, Watson M (2001). "Side-by-side Testing of Eight Solar Water Heating Systems" (PDF). United Kingdom Department of Trade and Industry. Retrieved 2007-08-04. {{cite journal}}: Cite journal requires |journal= (help) ^{[dead link]}

External links

The Solar Trade Association

[1] Martin C, Watson M (2001). "Side-by-side Testing of Eight Solar Water Heating Systems" (PDF). United Kingdom Department of Trade and Industry. Retrieved 2007-08-04. {{cite journal}}: Cite journal requires |journal= (help) ^{[dead link]}

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