Solar Thermal

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Solar Thermal Energy

Two ways are available to utilize the sun’s energy in the home: generating electricity using solar PV and generating heat through solar thermal technologies. 

These two methods might seem similar but are different in benefits, costs, and applications.

This article focuses on generating heat through solar thermal and shows you all you need to know about solar thermal.

What Is Solar Thermal?

Solar thermal is any technology that deals with converting the sun’s energy into heat. 

The heat generated through these technologies is useful for three purposes: hot water heating, space heating, or generating electricity.

Different technologies are also adopted to achieve these purposes, though they all leverage the sun’s power for generating heat and cutting back energy needs.

While a technology of solar thermal can convert heat to electricity, it’s worth knowing that solar thermal is quite different from solar PV. 

While solar PV uses the sun’s energy to generate electricity directly from the sun using the semiconductors’ photovoltaic effect, solar thermal generate heat from the sun but can later convert the heat generated to electricity, depending on its purpose.

Solar Thermal Power Plants

Most utility-scale solar projects utilize solar thermal to generate electricity. 

This method contains a steam-driven turbine. The energy from the sun is directed to a collector that heats up a liquid into steam.

Mirrors are involved to countertrade the sun’s energy to the collector, earning this technology the name ‘concentrating solar power’ (CSP). 

CSP plants are large and powerful projects that can produce up to 100 megawatts of power, covering around 10,000 residential installations.

Solar Thermal for Hot Water

The old way of heating up hot water in the house is using gas or electricity, which is not cost-efficient. 

With solar thermal, solar thermal lets you heat up your hot water using the sun’s energy without the need for gas or fuel. 

In this system, a solar collect collects the sun’s heat and uses it for hot water heating in the house.

The hot water storage tank stores this water and keeps the temperature high for later use.

Solar Thermal for Space Heating

Two options are available when considering solar thermal for space heating in the home: active and passive solar heating. 

Active solar thermal heating is a system where an array of solar collectors are installed on the roof like solar PV panels. 

These collectors collect the energy (heat) from the sun and send it to the house’s heat exchanger or a piping system that uses hot water to heat the house.

The second option, passive solar heating, is a method with no need to install any mechanical modules. 

This system collects the sun’s energy directly to heat the home. 

An example is installing south-facing windows that collect and store heat from the sun to heat the house’s ambient areas later.

Both active and passive solar heatings are efficient in homes in sunny areas with large heating demands. 

Using any of these methods reduces the electricity and fuel costs of homes and businesses significantly.

Since passive solar thermal heating works with special designs, it’s more cost-effective for new constructions, meaning that solar heating can be incorporated into the construction plan.

Solar Thermal Vs. Solar PV: Comparisons

Since solar thermal are further divided, it can be difficult to make a like-to-like comparison of the two technologies – solar thermal and solar PV. 

However, comparing the solar PV system with CSP – a similar technology – makes a fair comparison.

Though both solar PV and CSP help generate electricity for homes and businesses, CSP systems are more efficient than solar PV systems. 

However, since CSP generates a huge amount of power, they’re large projects that consume a lot of land areas. 

As such, this might not be a great idea for individual homes. Also, the technology is only suitable for certain geographical areas.

You can consider incorporating passive solar heating into your home design to offload your house electric demands for a lifetime. 

This option is also a great choice to combine with solar PV, taking out the best of your solar panels.

Offset Your Home Energy Use With Solar

How much of your home energy needs you can offload with solar depends on how much advantage of the solar you take. 

While you can leverage the sun’s energy for hot water heating and space heating, installing solar PV lets you enjoy the benefits of the sun further, reducing your energy and fuel bills.

Solar Thermal Collectors

Solar thermal collectors are of two categories: glazed and unglazed. Glazed solar collectors are a perfect choice for space heating. 

These collectors collect heat from the sun, transfer it into the house using a solar air panel and send the air back for re-heating. 

For this to happen, at least two penetrations are required. Most glazed collectors are designed for residential homes and work only when the air in the house is cooler than the collectors’ air.

The other type of solar collectors, unglazed solar collectors, function as an exterior wall to the building. 

These collectors absorb heat from the sun and send it to the house through the wall. 

Heat Storage for Space Heating

One common challenge often faced with solar heating is during the periods of the year when there is not enough sunlight. 

Since solar thermal for heating depends on heat from the sun, the system might not be efficient other than the summer. 

To combat this, seasonal thermal energy storage (STES) is introduced to store heat. 

The system is effective that it can store heat for months for heating the house all year round.

Common mediums used for STES thermal storage include boreholes equipped with heat exchangers, insulated surface water tanks, and large lined pits.

The best heat storage medium depends on how long the heat is to be stored. 

While some mediums are efficient at storing heat all year round (inter-seasonal storage), some are best for storing solar thermal heat just for days or even hours (short-term storage).

Inter-Seasonal Storage. The best mediums to store heat from the sun for months include specially-constructed pits and aquifers. 

These methods are great for storing heat generated with solar thermal for another season. 

Short-term storage. When considering storing heat for hours, thermal mass materials are a great choice. Thermal mass materials include water, stone, and concrete. 

The construction and placement of thermal mass storage systems should consider certain factors, such as daylighting, climate, and shading conditions. 

With proper construction and usage, thermal mass storage mediums help maintain comfortable temperatures for the whole day and reduce energy consumption.

System Designs

Solar thermal leverage the sun’s energy, meaning that efficiency depends on the amount of sunlight the collectors are exposed to. 

With this, the system’s efficiency is high during the time of the day when the collectors face the sun and reduces as the sun moves away from the direction of the collector.

A tracking system is put in place to maximize the amount of sunlight that the collectors are exposed to. 

This tracker tracks the sun’s movement and adjusts the mirrors, focusing the sun’s heat on the collectors.

However, it should be noted that introducing a tracker to a solar thermal system increases the cost and complexity of the installation. 

With this, it’s best to choose a suitable design that can help minimize these changes. There’re different designs to choose from, including the following:

Parabolic Trough Designs

Parabolic trough designs are one common tracking system in solar thermal technologies. 

This involves a parabolic trough directing the mirrors in the sun’s direction to concentrate the sun’s heat on the receiver. 

The parabolic trough is put in place to tilt east to west, following the sun’s direction. 

However, the system doesn’t have to move the mirror to meet up with seasonal changes in the sun’s direction.

Power Tower Designs

Power tower designs are another common but more complex solar tracking system where thousands of tracking mirrors are used to concentrate the sun’s energy on the collectors. 

A receiver is installed on the top of a tower placed at the center of the group of mirrors. 

Molten salt is placed on the receiver to be heated to over 1,000 °F (538 °C) and is then transferred to the thermal storage tank for storage. 

The heated salt, maintaining about 98% efficiency, is used to power a turbine to generate electricity. 

These are similar processes involved in standard coal-fired power plant, but this system leverages free solar energy for electricity generation.

The power tower design is more advantageous than the parabolic trough because it deals with high temperatures. 

With this, the thermal energy can be converted to electricity more efficiently than in parabolic designs. 

Heat Collection and Exchange

Solar thermal involves collecting and exchanging heat, and these are guided by five principles: heat gain, heat storage, heat transfer, heat insulation, and heat transport. 

These phases of solar thermal involve using exchangers. 

Copper heat exchangers are the most common heat exchanger used, going by their high thermal conductivity, mechanical strength, and resistance to water and atmospheric corrosion.

Any Grant for Solar Thermal Technologies?

The UK is gradually shifting from the old gas- and oil-powered boilers to renewable heating systems. Solar thermal is a great solution to this move. 

Today, there is a great surge in the number of people interested in solar thermal technologies, especially with the introduction of the Green Homes Grant.

Solar thermal is one of the many low-carbon technologies eligible for the Green Homes Grant.

This grant pays every eligible installation two-third of the total installation costs, which might be up to £5,000. 

This means that the owner pays one-third of the installation costs while the government pays the remaining two-thirds. 

For example, if a homeowner installs a solar thermal technology for heating his home and spends £6,000, he is expected to pay £2,000 while the government’s grant covers the remaining £4,000. 


A solar thermal system pays back in two ways: the government’s incentive and the advantage of using free solar energy to generate heat. 

While the government pays back two-thirds of installing a solar thermal system, generating heat from free energy from the sun helps save money and reduce energy bills.

RHI Return

It’ll also interest you to learn that other than the Green Home Grants, solar thermal systems are eligible for the Renewable Heat Incentive. 

This system pays a certain amount back to the amount of energy generated by the system. 

The payment is made quarterly and is recurring over a period of years.

Though you can apply for the two incentives, you might not have access to full payments under the two grants with a single installation.

If you’ve used your solar thermal system to claim the Green Homes Grant voucher, you’ll need to inform Ofgem when applying for Domestic RHI. 

With this, the amount you received with the Green Homes Grant is deducted from your Domestic Renewable Heat Incentive payments.

How Much To Expect From RHI with Solar Thermal

The payments to expect from RHI with solar thermal depend on the family’s size and the system installed. 

For example, a small system of 2m-square size for one person can generate around £1,000 over the 7 years, while a large system of 6-meter size for a family of 6 can generate up to £3,000 over the 7 years.

What of Homes with Less Hot Water Use?

The amount of hot water you use in your home doesn’t influence how much you’re paid on your solar thermal system since it is based on the installed system and not on hot water usage. 

However, how much you’re able to save with your system depends on how much hot water your family uses. 

A large family with higher hot water demand saves more money with solar thermal than a small family with less hot water usage.

Is Solar Thermal Worth It?

Solar thermal systems are a great choice for UK homes, going by their eligibility to the Renewable Heat Incentives and Green Homes Grant. 

However, it’s worth noting that the size of the family and the size of the system determines how much return is made.