When speaking about solar panels, it is important to distinguish between two main types:

1. Photovoltaic panels that produce electricity
2. Solar thermal panels that are used for heating purposes

Solar Photovoltaic Panels

Solar panels collect clean renewable energy in the form of sunlight and convert that light into electricity which can then be used to provide power for electrical loads. Solar panels are comprised of several individual solar cells which are themselves composed of layers of silicon, phosphorous (which provides the negative charge), and boron (which provides the positive charge). Solar panels absorb the photons and in doing so initiate an electric current. The resulting energy generated from photons striking the surface of the solar panel allows electrons to be knocked out of their atomic orbits and released into the electric field generated by the solar cells which then pull these free electrons into a directional current. This entire process is known as the Photovoltaic Effect. An average home has more than enough roof area for the necessary number of solar panels to produce enough solar electricrity to supply all of its power needs excess electricity generated goes onto the main power grid, paying off in electricity use at night.

Grid-Connected vs. Stand-Alone PV Systems

Grid-connected systems make use of the local utility grid to ensure you are never without electricity. If your domestic solar panel system generates more electricity than your household requires in a day, this surplus energy can be exported back to the national grid. On the other hand, if you need more electricity than your solar panels have generated, the grid can supply this.
Stand-alone PV systems are not connected to the grid, but instead charge a solar battery system. These batteries store the electricity generated by your panels. To operate your appliances, the stored electricity from these batteries will be used. Stand-alone systems are used in areas that cannot be connected to a grid, and are typically more expensive than grid-connected systems because solar batteries are still quite costly.

Solar Battery Storage Systems

Solar batteries store electricity generated by the sun’s energy and allow you to use it during the evenings when your solar panels are not generating electricity. You can make use of solar batteries whether you are connected to the grid or not.

Solar Thermal Collectors

Both solar thermal and thermodynamic panels can be used for space heating, but most commonly they provide the domestic hot water (DHW) needs of households.

How do solar thermal panels work?

Solar thermal panels convert solar energy into heat that is then used to warm the water within a cylinder. This is contrary to solar PV panels which convert the sun’s energy into electricity. There are two different types of solar thermal panel: flat plate collectors and evacuated tube collectors.

Flat plate collectors: In terms of appearance, flat plate collectors most closely resemble solar PV panels. Covering the metal tubing, which contains the fluid heated by the sun, is a flat absorber plate that’s dark in colour. The benefit of having a flat plate covering the tubing is that it prevents heat from escaping.

Evacuated tube collectors: Exposed tubing separates evacuated tube collectors from flat plate collectors. Several tubes run side-by-side in a parallel line. They can either be direct flow which means that liquid in the tubes circulates down to the immersion heater in the hot water cylinder. Or, in the form of heat pipes where the fluid within the tubes evaporates and transfers the heat to the pipes in the cylinder.

Best solar thermal panels: flat plate or evacuated tube?

If how the solar thermal panels will look on your roof is your main concern then flat plate collectors are arguably the better option. Thanks to their flat dark-coloured appearance, flat plate collectors will more seamlessly fit in with the roof. Additionally, as they resemble solar PV panels, they have a familiar feel about them.

In terms of performance, evacuated tubes are capable of delivering higher year-round efficiency. When solar generation is at its peak, during the summer, both types of solar thermal panels will be equally as efficient. However, during the winter months, when sunlight is at a premium, evacuated tubes are capable of providing higher levels of hot water. This is thanks to better all round insulation.

So while flat plate collectors might be easier on the eye, evacuated tube collectors are arguably the best solar thermal panels.

Source: Green Match / Mr Solar / Solar Guide

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