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How do solar thermal collectors work? A guide

The sight of solar panels on rooftops around the UK is becoming more and more common. According to GreenMatch, we are installing solar panels faster than any other European country. These solar panels work to convert sunlight into electricity, and then solar thermal collectors convert the sun’s energy to then power the heating of a property.

If your customer wants to use the sun’s power to save money on their energy bills, installing a solar thermal system will do the job.

What is a solar thermal collector?

Solar thermal collectors absorb the sun’s rays and change them to heat to make hot water. It’s an eco-friendly way to heat water for use around a property. It’s important to understand that while both solar panels and solar thermals gather energy from the sun, they are two very different technologies.

The purpose of solar thermal technology is heating water. It’s used for heating water in domestic and industrial buildings and is especially popular for heating swimming pools.

thermal collector

How does a solar thermal collector work?

A solar thermal system uses roof-mounted solar panels that are called solar collectors. They use the sun’s energy by working with a boiler or immersion heater. In most domestic systems, the sun's heat energy increases the transfer fluid's temperature in the collector tubes. This fluid usually combines glycol (antifreeze) and water to prevent the water from freezing.

The heated water from the solar collectors is then pumped to a heat exchanger, which is integrated into the water tank in the building. The heat from the exchanger will then heat the liquid inside the tank. After the liquid releases the heat, the water travels back to the tubes for reheating. A controller will circulate the fluid to the collector when enough heat is available.

Even though the collector is the main component in a solar thermal system, there are other vital elements to be aware of during solar thermal installation.

Solar thermal system components

A standard solar water heating system will contain the following components:

Collectors – These are the solar thermal panels used to capture the sun’s energy. They are typically installed on the property’s roof using brackets and frames. However, in some cases, they are mounted on the ground. Solar thermal collectors come in two types: flat plate or excavated tubes.

Heat transfer fluid – This is the fluid that moves the heat from the solar collector panel to the hot water tank. It can be anti-freeze, water or a mixture of the two.

Twin coil water cylinder – A twin coil water cylinder is used as there are two heat sources. The solar circuit is connected to the lower coil, allowing the collectors' heat to be transferred as efficiently as possible.

Pump – The pump is usually powered by mains electricity. Its job is circulating the heat transfer fluid between the collectors and cylinder.

System control panel – Regulates the pump and provides information on the system’s performance. It also highlights any potential faults.

Insulated pipes – These are fitted between the panels and the water cylinder. It’s known as flow and return piping.

Heat flow generation meter – Enables your customer to claim Renewable Heat Incentive (RHI) payments by measuring the system’s output.

flate plate collector

Different types of solar thermal collectors

Solar thermal hot water technologies can be categorised based on the type of collectors:

Flat plate solar thermal panels

The most common type of solar thermal collector. Flat plate solar collectors consist of a flat absorber plate, a transparent cover and insulation. The absorber plate is usually dark coloured and mounted in an insulated metal frame. Insulation surrounds the frame to retain the heat.

The transparent cover is usually double glass and encases metal tubes. It provides a space of air that works as insulation, allowing solar energy to pass through the cover and reducing heat loss from the absorber.

They are ideally suited for low-temperature systems, such as floor heating.

evacuated tube collector

Evacuated tube collectors

An evacuated tube collector has rows of insulated glass tubes. The tubes contain a sealed copper heat pipe, sending the heated water to the tank. The vacuum surrounding the tubes' exterior reduces the risk of heat loss, contributing to the technology’s impressive 70% efficiency rate.

They last around 25 years and have a high performance, so are more expensive. However, they require less roof area for the same output. For an average household, 20-30 tubes are usually enough to meet hot water demand.

Benefits of solar thermal collectors

From reducing a home’s carbon footprint to cheaper energy bills, let’s look at some of the key benefits of solar thermal collectors:

No running costs – Once you’ve installed a solar thermal water heating system, there are no running costs for your customer.

Easy to maintain – Aside from professional servicing every 3-5 years and an annual check for leaks, solar water heating systems are virtually maintenance-free.

Green credentials – The most important benefit is that solar thermal collectors use free, renewable solar energy. An average household can reduce CO2 emissions by up to 400kg per year.

Sustainable to high temperatures – Solar thermal collectors can bear high temperatures. Flat plate panels can attain temperatures as high as 200°C, while excavated-tube collectors can reach temperatures as high as 295°C.

Cheaper energy bills – An average solar water heating system can provide 40-60% of the hot water for a four-person household. This includes hot water for hot taps, showers, and baths. Of course, the time of year will also impact how much water the technology produces. The winter months will only contribute a small amount to water heating, as the days are shorter and the sunlight is weaker.

What happens to the heated water not immediately used in a solar thermal collector?

A solar thermal collector’s efficiency is much higher in summer than in winter. Yet the demand for hot water is at its lowest during the long, hot summer days. This often means there is more hot water capacity than is needed. 

Solar thermal technology is designed so the heated water is not wasted. It is stored in a separate tank for preheating or a regular water tank until needed. The regular boiler or an immersion heater will top it up if additional heat is required.

This highlights that you still need a boiler, which is one of the disadvantages of this technology.

Disadvantages of solar thermal collectors

Weather dependent – A solar water heating system is unlikely to deliver enough water for the whole year. When clouds hide the sun, it’s unable to do its job, which is why you would still need a boiler.

Won’t supply all appliances – Many modern houses have electric showers, cold-fill washing machines and dishwashers. However, solar water heaters aren’t compatible with these types of appliances. For households that don’t use much hot water other than this, having a solar thermal system installed might not make a difference.

They are also only suitable for some combi-boilers. This is because the water to a combi-boiler is supplied directly from the mains and generally will not accept pre-heated water.

Sufficient space – The property you’re working on requires enough space for a solar cylinder and enough sunlight to cover heat demand. Consider the hot water requirements of the household or business. A general rule of thumb is to allow 1m² of solar collector area for each person living in the building. 

Limited use – While they can heat water, solar thermal collectors cannot generate electricity or heat rooms.

Cost to install – The average cost of solar thermal installation is between £3,000 and £5,000. However, it’s important to think about the long-term gains. Not only does it save money on bills, but your customer will also be eligible for the RHI, which supports them with yearly payments.

Solar collector panels rely on perfect weather conditions but can be the right long-term choice for a home or business. They are an environmentally-friendly and sustainable energy source that can save your clients money over time while being kinder to the planet

The information on this website is intended to be for general guidance – however every project is different which could affect the suitability for your particular project or circumstances. References to a product, service or material should not be considered a recommendation or any indication of fitness for any specific purpose. Where applicable work must be carried out by a qualified professional. Seek appropriate specialist advice if needed and always follow manufacturer’s instructions advice and ensure compliance with any applicable laws or regulations.

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