How to Combine Renewable Energy Sources

Sustainability expert Nigel Griffiths gives his inside advice on which renewable technologies make the perfect partners for heating and hot water
Build It magazine expert Nigel Griffiths
by Nigel Griffiths
19th September 2019

A key goal for many self-builders is to create a comfortable home with minimal running costs – and critical to achieving that is getting the heating strategy right. There’s a host of renewable energy technologies to choose from, so how can you select an effective combination that will help to cut your energy bills and reduce your household’s environmental impact?

There are three main functions to consider when it comes to creating a comfortable living environment, namely water heating, space heating and cooling. In this article, I’ll look at the key principles of linking different renewable resources to effectively deliver these services.

Selecting the best system for your project will depend on your budget, eco priorities and how far you want to go in reducing running costs.

Combining renewable energy sources – the basics

The two key elements to bear in mind when combining renewables are seasonality and time of use. With the former, the trick is to find heat sources that will complement each other – so one is available when the other is not. A basic example would be to rely on sunshine in the summer months, and then set cosy log fires in winter (albeit an open fire isn’t exactly the most efficient use of fuel).

Time of use refers to the fact that renewables are at their most effective in terms of reducing carbon emissions when they preclude the need to draw energy from the national grid. So it makes sense to combine systems in such a way that power is used when it is being generated – eg when running heat sources off renewable electricity.

Latest Renewable Heat Incentive Tariff rates

Applications submitted beforeBiomass boilers & stoves (p/kWh)Air source heat pumps (p/kWh)Ground source heat pumps (p/kWh)Solar thermal (p/kWh)
April 20186.54p10.18p19.86p20.06p
January 20184.28p7.63p19.64p20.06p
July 20186.74p10.49p20.46p20.66p
June 20196.88p 10.71p20.89p21.09p
March 20196.74p10.49p20.46p20.66p
October 20186.74p10.49p20.46p20.66p
September 20196.88p10.71p20.89p21.09p

Of course, it’s also possible to combine sustainable resources with fossil fuels or grid electricity – and in most homes both will still have a role to play. Furthermore, if you have natural gas available then you may well find this is among the cheapest routes to delivering space heating, and almost certainly the most affordable to install.

Stovax View stove

The Stovax View 8HB is a woodburning and multifuel boiler stove capable of outputting 4.9kW of heat to a room, and 8.2kW to water

Reducing your home’s environmental impact

In terms of carbon emissions in use, there’s little to choose between modern gas boilers and most heat pumps – but the latter have a much higher installation cost and embodied energy (that used to manufacture the appliance).

That said, if reducing your carbon footprint is a top priority, then you could consider a hybrid setup that combines a gas boiler and air source heat pump (ASHP). This type of system uses intelligent technology to select the best heat source given the conditions (and costs).

So typically, it would rely on the ASHP in the ‘shoulder months’ either side of winter, when air temperatures are reasonable, but switch to gas in the colder periods of the year when the heat pump would be less efficient.

Incidentally, ground source heat pumps (GSHPs) do not suffer so much from seasonality problems as their energy collecting loops are buried in the earth where temperatures are, in theory, more stable.

The best renewable energy setups

If you’re off the mains gas network or connection costs are prohibitive, then switching to a dedicated sustainable tech solution is likely to become a more attractive option. Here are the key combinations to consider:

Water heating only

The term domestic hot water (DHW) is used to describe that used throughout the house for washing and cleaning. There are a number of renewable options for supplying DHW, but if you’re aiming for a reduced carbon footprint then there’s one that stands out in particular – namely using solar thermal panels alongside a woodburning stove with an integral back boiler, as part of a stored hot water setup.

With this combination, the stove will be providing both hot water and an element of space heating in the colder months when there’s little or no solar resource; while in summer the thermal panels will provide most of your DHW.

There will be times in winter when the sun shines strongly and the solar collectors can operate, so a good controller is needed to make best use of this. Equally, there will be occasions in summer when the sun is shy and the woodburner is off, which means a backup source of energy is required.

This standby resource usually takes the form of an electric immersion heater. While this part of the setup may be fairly expensive to run, the demand on it should be very low. With this exception, the combination of solar thermal panels and a woodburning stove provides near carbon-neutral hot water – so it’s a great eco choice.

Biomass boilers running on pellet or chip-based fuels could do exactly the same job as the stove in this scenario, and are less effort to fire up for a short period in summer if you need to heat the cylinder.

Low-energy self-build home

House with Worcester Bosch renewablesKey technology: 11kW ground source heat pump and solar thermal panels, both from Worcester, Bosch Group

When  self-builders Ian and Joanna Stewart found a plot of land just outside Kelso in the Scottish Borders, they were determined to seize the opportunity to create their dream eco home.

“We had always been drawn to the location,” says Ian, who is a Worcester, Bosch Group accredited installer at James Stewart & Son. “The village is small and friendly – but the plot’s remote position brought the issue of being off the mains gas network.

During the planning stages of the project, the price of oil seemed to be rising on a daily basis, so we felt we had to look at the alternatives. We also strongly believed we should be looking for ways to reduce our carbon footprint and do our bit for the planet.”

The couple planned to make this their home for life, so identifying the right energy sources was critical. “I spoke to Worcester in depth about the merits of combining heat pumps and solar thermal for our self-build,” says Ian. “It was then a case of emailing over the CAD drawings of the house layout and overall scheme, along with my thoughts about what we wanted to achieve. Worcester designed the whole system for us, right down to the number of collector pipes that would be needed to supply the ground source heat pump (GSHP).”

Worcester recommended an 11kW Greenstore GSHP working in tandem with its Greenskies solar water heating system (as a guide, a 4m2 array would deliver around 4kW of heat input). Underfloor heating has also been fitted on both storeys to complement the GSHP. “The combination of these products satisfies all our heating and hot water requirements using free energy,” says Ian.

For a similar installation, expect to pay around £3,000-£5,000 for the solar thermal system and £13,000-£20,000 for the ground source heat pump. Worcester calculates that running costs for a project of this nature would be around £1,500 per year.

Worcester solar panels and controls

Space heating

Again, there are multiple renewables strategies to choose from here – but when it comes to combining them effectively, you’ll need to take a heat storage approach. Fundamentally, this means fitting an accumulator at the heart of your system.

This is essentially a large, very well insulated hot water tank that includes multiple tapping points and coils to allow for several inputs – such as from a biomass boiler or heat pump working alongside a solar thermal array. Installing this kind of setup won’t come cheap; you can expect to pay several times the cost of a gas combi boiler to fit a solar water heater, air source heat pump (ASHP) and an accumulator.

So if you already have access to mains gas, this route is only really worth considering if the extra investment will be outweighed by savings in the running costs (either financially or in terms of carbon emissions, depending on your motivations). The Renewable Heat Incentive may assist with this.

A wood pellet biomass boiler would do the same job as an ASHP at similar cost and lower emissions. In either case, you would use the same system to provide your DHW via a heat exchanger in the accumulator – so there’s no need for separate domestic hot water storage.

You’ll need to allow sufficient space to accommodate the heat pump or biomass boiler equipment and its fuel, as well as the accumulator (1,000L is a common size for domestic installations). Sometimes, heat pumps are combined with an intermittent source such as a woodburning stove (as opposed to a biomass boiler, which is always online).

Renewable technology eco renovation

House with Viessmann solar panels

Key tech: 13kW split air-to-water heat pump, 2.8kW solar thermal panels and 3.75kW solar PV array, all from Viessmann

When the Jenkinson family noticed others in their community were starting to benefit from energy savings and reduced bills by fitting renewable heating systems, they were keen to explore the options for their characterful country home in Devon. The house had been running on an old oil boiler, which was just 65% efficient, so there was huge scope to upgrade to a better-performing setup.

The Jenkinsons engaged Viessmann partner Lenwood Energies to investigate the options, and the company came up with a package that would be perfect for the young family’s needs. Hot water would be in use throughout the day, so specifying a 2.8kW Vitosol 200-T solar thermal array (which harvests energy during daylight hours) was a natural part of the solution.

This was installed alongside a 13kW Vitocal 242-S split air-to-water heat pump, which powers 10 newly-installed fan assisted radiators and features an integral 220L cylinder for solar heating. The Viessmann system is governed by the company’s Vitotronic 200 controls, which intelligently decide which energy source will deliver optimum performance at any given time. The project also features a 3.75kW Vitovolt solar PV array.

The heat pump has an average running time of 105 hours per week in the winter months, and the property is now entirely heated by renewable energy. “The household now saves nearly £1,500 per annum on its utility bills and the installation costs will be recovered in well under eight years,” says Chris Lenehan from Lenwood Energies.

Viessmann solar thermal and heat pump for eco renovation


Every cooling system requires electricity to run, but most routes to generating this renewably are intermittent. Solar photovoltaic (PV) panels only work effectively when there’s good daylighting, for example, while wind turbines rely on a decent breeze.

If you are lucky enough to have a spring-fed source of water power then the situation changes; but this is rare. Many locations are not suitable for wind power – but most properties can accommodate some form of PV.

As you’d imagine, overheating is typically only a problem when the sun is shining – so household cooling requirements are likely to coincide with the part of the year when solar photovoltaic (PV) panels can perform to their maximum. For this reason, PV is an ideal partner for cooling systems.

Many heat pumps can also provide cooling as the key component of an air conditioning system. As the time of use matches up well with the time of generation, combining PV and an ASHP for this purpose means you won’t be imposing an additional load on the grid – so it can deliver genuine carbon emission savings.

A word of warning, though – don’t assume that using a PV and ASHP together will help to achieve a reduced net impact in terms of your costs and carbon footprint for space heating. The sun won’t be shining so much during heating season, so the time of generation doesn’t correspond with when the energy is needed most.

Image (top): This self-built eco home features Nu-Heat underfloor heating powered by an air source heat pump – an ideal combination

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