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.

Zero carbon solar controller

A "zero carbon" solar controller uses solar electricity produced on-site in a photovoltaic (PV) panel to run the pump that delivers the solar-heated water to the hot water store. The best known of these in the UK market is the Solartwin, a simple system with flat plate collectors and a pump driven directly by a PV panel.^[1]

The assumption justifying a zero carbon controller is that the energy required to operate the pump and valves is a significant fraction of the useful heat energy delivered by the system, and so avoiding the need to supply this energy from fossil sources is a direct benefit.^[1] As the energy required to run such a pump is actually small in comparison to the heating power of the system, then such an assumption is tenuous at best. In the case of the Solartwin, the most practical benefit is the resultant simplicity of the overall system. Rather than using complex algorithms based on store and panel temperatures, the pump is driven directly bby the PV panel: when the sun shines, the pump runs.^[1] In practice this is as efficient a practical control algorithm as most others achieve and has obvious advantages for reduced system complexity.

A further advantage to the Solartwin approach is that the system may be operated as a "drain down" panel. When not in active use, the fluid in the panels is allowed to drain down into the indoor reservoir, emptying the panel. When the sun shines, the PV panel powers the pump and the thermal solar panel is refilled and circulated. This makes the system self-protecting against frost, event without the need for glycol or anti-freeze additives to the system.^[1]

A disadvantage to the Solartwin is that the pump stops immediately after the sun is occluded. With vacumm 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.

A zero carbon solar controller (the controller alone, not the pumps) may contain an electricity store^{[citation needed]} in addition to the same components as a normal solar controller. This electricity store is usually in the form of supercapacitors, since these have a much longer life than batteries. The electricity store allows the controller to remain powered and display temperatures at night when there is no sunlight.

The benefits of a zero carbon solar controller are that the carbon clawback associated with conventional solar thermal panels is avoided, provided that the pump used is also powered by photovoltaics.^[2] This sustainability benefit comes with a slight panel performance reduction of in the range of 1-10%. This relates to the times when the panel may be slightly hotter than the water store but when there is not enough power from the PV to turn the pump on. This happens mainly on hot days in summer, at times when hot water is likely to be in excess, so this potential reduction may not be as significant as it may first appear.

Currently (2007) a minority sub-technology within an already minority technology; it is however possible that the global solar thermal industry will start to adopt zero carbon controllers more widely in the future. 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. 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.

References

^ ^a ^b ^c ^d http://www.solartwin.com/solartwin-features/basic/solar-pump
^ 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

[Solartwin,_pump-1] ttp://www.solartwin.com/solartwin-features/basic/solar-pump

[2] 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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