Solar - Thermal


solar-thermal-videos - done reading

What is a solar-thermal system?

It is a proven renewable-energy technology that enables the rays of the sun to heat your home’s hot-water supply. Unlike solar-photovoltaic panels, which use the sun to generate energy (electricity), a solar-thermal system, instead of generating energy, you’re saving it. When compared to solar-electric panels, solar-thermal is a lower-cost option….

What are the common components of a solar-thermal system?

Solar thermal energy systems heat indoor air or water, and they are also used for air-conditioning. Solar collectors trap the sun’s rays to produce heat. Most collectors are built in the shape of a box, frame, or wall, and some are a whole room. Most contain these parts: (1) clear covers that let in solar energy; (2) dark interior surfaces (absorbers) that soak up heat; (3) insulation materials that prevent heat from escaping; and (4) vents or pipes that carry heated air or liquid from inside the collector to the place where it is used.

When is using solar PV to heat water not a smart idea?

Using solar photoelectric (solar PV) panels to heat water is very inefficient. It is far better to use dedicated solar water heating panels. However, if you have an array of solar PV which is exporting electricity to the grid for which you are being paid only a few pence per unit, it makes financial sense to use that electricity to heat water since a few pence per unit is a lot cheaper than the cost of buying electricity or gas for water heating.

While virtually all multi-kW solar PV arrays are supplied with a digital display unit to show the amount of power being generated, it is very difficult for the non-electrician/expert to use that (or to start cutting into cables etc) in order to detect when the power generated is above a certain threshold. If for example you have a 9kW solar array, and want a 3kW immersion element to turn on in a hot water tank when the solar power generated exceeds 4kW, things could be quite complicated and expensive. In order to avoid the need to mess around with the solar array and associated cables and electronics, we have come up with a simple device which operates completely independently of the solar array, but which will turn on an immersion heater whenever solar output is high enough. A light dependent resistor (aka LDR or light detector) such as that pictured above is positioned facing in the same direction as the solar panels - i.e. when the panels are directly facing the sun, the LDR will also be facing into the sun. The LDR is connected to a simple controller which does the following:

When the user knows that the solar panels are generating just a little above the target power level (from the readout on their digital display), they press a button on the controller. This calibrates the controller so that whenever the LDR measures that it is brighter than this user-set threshold, the controller will know that the power being generated by the solar array exceeds the target and therefore turn on the immersion element. When the LDR measures the light level to be lower than the threshold, the controller will know that the power being generated by the solar array is less than the target and therefore will turn off the immersion element.

The immersion element itself is turned on and off using a relay switched by the controller. In order that the immersion element is not turned on and off rapidly and repeatedly when the light level is very close to the user-set threshold, we have added some timers to the controller. The immersion element is only turned on after 5 continuous seconds of the light level being measured to be greater than the threshold. Subsequently, the immersion element is only turned off again after 2 continuous seconds of the light level being measured to be below the threshold.

When choosing the threshold level of power generation at which the immersion is to be turned on it is essential to consider the background level of power used in the house - for example, the fridge, WIFI routers, computers, lighting, and devices on standby will be drawing power all the time. At least 1kW should be added to the power rating of the immersion element - e.g. use a threshold of at least 3kW if the immersion element is rated at 2kW. The key thing is not to find yourself importing expensive electricity to power the immersion!


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