Solar water heating in the UK for homeowners: solar thermal vs solar PV

The future of hot water – what makes solar hot water systems appealing?

A lot of energy used in homes is to heat water; in fact, it’s about 17% of the total household energy consumption in the UK. Did you know that across Britain, residents use about 840 billion litres of water each year, and spend about £2.3 billion on heating water each year?

One way of reducing costs and carbon emissions related to domestic hot water is by switching to a solar hot water system.

Solar water heaters (based on the solar thermal technology) use collectors on the roof to use the sun’s energy to warm water for storage in a hot water cylinder. They serve as a direct alternative to conventional water heating systems, such as gas water heaters, tankless coil water heaters, heat pump water heaters and electric water heaters, all of which use either gas, oil, or electricity to power them. The technology is not very complex and there are no running costs associated with a solar hot water system, which explains their growing popularity. According to MCS, around 43,667 homes across the UK have solar water heaters installed, as of August 2023.

How do solar hot water systems work?

Typically, solar collectors, controller system, heat exchanger, a backup heater, and most importantly, a thermal storage (such as a solar hot water cylinder) are the components that together make up a solar hot water system.

These systems use panels or evacuated tubes called solar collectors, which are filled with a mix of water & glycol, to gather solar energy and convert visible light into heat.
There are two types of solar water heating collectors, evacuated tube collectors and flat plate collectors:

 

Flat plate collectors

A flat plate collector (FPC) is a type of panel that converts solar energy into thermal energy –  a cheaper form of energy that uses water as its operating fluid. The collector’s heat-absorbing plate is exposed to sunlight and when the sun’s rays strike the flat plate surface, some of the energy is converted into heat and the temperature of the flat plate solar collector rises.

 

Evacuated tube collectors:

An evacuated tube collector (ETC) collects & stores solar heat through a series of vacuum-sealed tubes. ETCs are commonly utilised in commercial and industrial solar thermal systems because they capture more solar energy than FPCs. They are also commonly used in domestic solar hot water systems since they may produce more hot water than flat plate collectors.

Both collectors, though, are designed to maximise solar radiation absorption & and have great thermal efficiency. Also, both can be linked to storage tanks, heat exchangers, and other heating system components.

Potential cost and carbon savings

Solar is a clean source of energy and a solar water heating system lasts for about 20 years with proper installation and regular maintenance – that’s roughly five to ten years longer than conventional water heaters.
These systems typically designed to meet around half of your hot water needs.
The savings will vary depending on the time of year, with systems delivering nearly all your hot water in summer but less in the winter.
However, a well-insulated system can minimise heat loss and continue to provide hot water in cold winter months as well.

The following savings are based on a 4m2 system and vary depending on the user. This is field testing data from the Energy Saving Trust and Microgeneration Certification Scheme (MCS) methodology.

Then there’s carbon savings too. Simply put, the closer we can get to 100% of our hot water coming from solar systems, the more we can reduce our carbon footprint. Also, solar water heaters contribute to lesser greenhouse gas emissions (lower onsite emissions). This shift helps reduce carbon footprints and is a great way to go green and contribute to a safer environment.

How can UK homeowners benefit from grants for solar thermal installations?

If you’re a homeowner, you can take advantage of the ECO4 government-funded initiative, which started in July 2022 and will run until March 2026, if you live in England, Scotland, or Wales and fulfil the required criteria.
The scheme can save you up to 100% of the cost of new solar panels, as long as they power a heating system, and not electricity for other purposes, such as appliances. An average three-bedroom household will save about £7,860 on new solar panels through this scheme.

Also, the Home Upgrade Grant scheme in England can hand out grants of up to £10,000 for home improvements including solar panels. On average, a local authority will cover 87% of the cost of a homeowners solar panels, but could rise to 100%.

Some considerations with solar thermal systems:

i) Intermittency of energy source
Since the solar thermal system is dependent on sunlight for heating, it is only functional when the sunshine is abundant. So the energy production is fully dependent on the weather or season. The performance is best in areas with high solar radiation, clear skies and low temperatures. In colder and cloudier regions, such as the UK, this solar-dependent system may suffer from technical challenges due to intermittency of energy source.

ii) The need for thermal storage for hot water
In addition, the solar water heating system needs to be connected to a thermal storage component, a big hot water tank or a cylinder, which means that precious house space is used up, typically an entire cupboard, and there is intricate pipework involved. The issue is that hot water cannot be stored for a long period without significant standby heat losses, which is especially concerning with these systems as they are fully dependent on the sun’s heat.

A solar thermal alternative to harnessing solar: a peek into photovoltaics

While solar thermal system uses the sunlight to generate heat, a solar photovoltaics system (solar PV) works on the photovoltaic effect – photons (light) strike the semiconductor material, generating the release of electrons. Simply put, solar PV panels convert sunlight to electricity. A PV system reacts to any light during the day, even if the sun is obscured by clouds, whereas solar thermal relies on sunlight to heat the water in the collector. The system also has a much longer lifespan than solar thermal, since photovoltaics don’t have any moving parts unlike the thermal panels, and show no degradation for over 40 years.
In essence, there are two types of residential PV systems: grid-connected and off-grid. The first ones offer the advantage of ‘feeding the grid’ (sending electricity back to the grid), which has monetary incentives as well (read: Smart Export Guarantee (SEG) | Ofgem).
The case with solar PV-generated hot water systems, though, is that either a) they don’t generate enough electricity to meet household demands (such as domestic hot water) or b) generate excess electricity above the demand and export it back to the grid.
In this case, a solar PV diverter switch (which costs up to £400) can help redirect the surplus electricity to power the immersion heater in a hot water tank, or a heat battery for hot water, which also offers other advantages over a hot water tank, such as:

Saving space:
Thermino heat batteries are made of our patented Plentigrade phase change material. This allows storing more energy per unit volume compared to any other material, which makes them both highly energy dense and extremely compact, about a quarter of a size of an equivalent hot water tank. And because they have a compact cuboid shape, they can easily fit into tight spaces – from kitchen work tops to under stairs. And whether it’s a house, new build apartment or social housing flat, space is always at a premium in Britain’s homes (£187-£729/sq.ft depending on where you live in England).

Ultra-low heat losses:
The heat batteries are also vacuum-packed with insulated panels, ensuring very low energy losses compared to a hot water cylinder, which also amount to energy savings. Efficiency in heating and minimal heat losses are particularly important in the thermal storage component of a solar hot water system.

In case where solar PVs don’t generate enough electricity (they do have much lower efficiency than solar thermal system), the PV system can be paired with a heat pump, which typically has a high COP (up to 3-4), and the efficiency deficit can be offset.
And Thermino xPlus range of batteries work with both heat pumps and solar PV, which means your hot water usage can be provided entirely by free PV-generated electricity (especially in summers) or supplemented by highly efficient heat pumps, consuming very low electricity units in periods of intermittency. The greater advantage is that in this set up, the onsite carbon emissions can come down to zero.
As a homeowner, you may also be eligible to benefit from various schemes by the UK government incentivising solar PV installations.

 

Over 25,000 Thermino batteries have been installed worldwide both in residential and commercial spaces. If you’re a homeowner looking to fit this in your house, find a Sunamp-approved stockist or installer.
If you’re a housing provider or a home developer interested in installing our heat batteries in your project, get in touch with us here.