Blog: Biomass Heat: The Seasonal Solar Storage Technology

By Bruno Prior, MD Forever Fuels and WHA Director.

As the proportion of renewable energy in the UK increases, the question of storage becomes more important. This is not just an issue for electricity. In fact, it is more challenging to match the production of renewable energy to the demand for heat than to the demand for electricity.

Demand for electricity follows fairly predictable patterns. There are two peaks each day at breakfast and tea-time, with a plateau in the middle of the day and a trough at night. Weekend demand is a little lower than during the working week. And demand is somewhat higher in winter than in summer.

Demand for heat depends on the weather. It varies far more than demand for electricity from summer to winter. It also varies more from one year to another. And it varies dramatically from day to day and week to week. Weather patterns do not follow convenient diurnal cycles.

The total demand for heat is also much greater than the total demand for electricity. The disparity was illustrated in a chart produced by Dr Robert Sansom of Imperial College.[i]

Biomass Heat: The Seasonal Solar Storage Technology

The chart shows synthesised half-hourly heat demand (in red) and half-hourly electricity demand (in grey) in 2010. The heat demand is synthesised because there are no statistics available for national half-hourly heat demand. Dr Sansom developed a technique to estimate half-hourly heat demand, based on gas and weather data.[ii] The methodology and data look credible and have been widely relied upon in policy circles.

In the case of electricity, the challenge of balancing supply and demand consists mainly of smoothing intermittent output to match predictable demand. In the case of heat, if intermittent forms of renewable energy are used, there is a double challenge, matching variable energy production to heat demand that also varies widely but not in sync with the production.

There is not much that can be done about the weather and the patterns of heat demand. The scale of the challenge to match supply and demand for heat, therefore, depends on the balance of technologies used to supply the heat. Dispatchability is important for electricity but indispensable for heat.

The two central planks of the British government’s plans to decarbonise heat are:

  1. Biomethane injection into the gas grid, and
  2. Electrification of heat


Biogas needs to be produced continuously. We shape our natural gas supply to the seasonal demand largely by increasing the flows when they are needed (a) from our declining continental-shelf reserves, and (b) from imported natural gas, whether that is via the connectors to Norway and continental Europe, or through the LNG terminals.

Our gas storage is relatively limited: sufficient for around 6% of annual demand, or around 7 days of peak winter demand. And the maximum rate that that storage can release its gas is only sufficient to meet 38% of peak winter demand on cold days.

As the supply of biogas cannot be varied like the supply of natural gas, an increase in the share of biogas in the grid would require a matching increase in the amount of gas storage. That storage would not just be for short-term variations, but the much larger capacity needed to store gas produced in summer until it is needed in winter. The costs of long-term gas storage have been estimated at around £200 – 400 million for 500 million cubic metres if suitable depleted gas fields can be converted. That is enough for just over one day of peak winter demand.

In any case, the Government may be looking for a significant increase in biomethane relative to current levels, but relative to our total heat demand, the intended volumes of biogas are not the biggest contributor.


Electrification is much the more significant part of the decarbonisation plans. The core scenario of the models on which the current Heat Strategy is based assumed that heat-pump systems (either standalone heat pumps or hybrid gas boilers including heat pumps) would supply more than half our heat in 10 years’ time.

The strategy depends on the use of low-carbon electricity. But little attention has been paid to the correlation between heat demand and the production patterns of low-carbon electricity.

We downloaded the Met Office’s hourly temperature readings from its stations around the UK,[iii] and Elexon’s half-hourly data for Actual Aggregated Generation per Type (report B1620),[iv] for the calendar year 2016. We used an approach based on Dr Sansom’s methodology and the Met Office data to calculate synthesised hourly heat demand figures for 2016. For each hourly period in 2016, we compared the synthesised heat demand with the aggregated generation from onshore wind, offshore wind and photovoltaic electricity.

To illustrate the different relationships between the weather, heat demand and production of renewable electricity, we calculated the average of these hourly figures at different temperatures, from less than 0°C to more than 18°C, in 2°C bands. To make it easier to compare the trajectories, the synthesised hourly heat demand is plotted against the left axis, and the electricity generation figures are plotted against the right axis.Biomass Heat: The Seasonal Solar Storage Technology

As one might expect, photovoltaic electricity has a high inverse correlation with heat demand. In other words, PV electricity is generated when we don’t need heat and not generated when we do. It could make a good power source for cooling, but it is a hopeless source of electricity for heating.

Wind generation does not have a significant correlation with heat demand, either positively or negatively. That means that wind output is sometimes high and sometimes low when heat is needed, and it is also sometimes high and sometimes low when there is little heat demand.

Noticeably, wind output is low during periods of peak heat demand, when the average national temperature is below 4°C. So is solar. A heat electrification strategy would have to ensure that spinning reserve was available to meet these periods of low renewable-electricity generation and peak heat demand.

But if we need a significant increase in fossil-fired spinning reserve to supply the electricity for electrified heating when the weather is cold, electrification isn’t contributing to decarbonisation in the way that the plan intends. Decarbonisation through electrification demands that we need to use low-carbon electricity to produce the heat when it is needed, i.e. when the weather is cold.

And we must increase the low-carbon electricity in proportion to the additional demand created by the electrification of heat. Our planned low-carbon electricity is already committed to the decarbonisation of existing electricity uses (e.g. lighting and appliances). To the extent that we electrify heat, we need to go beyond the planned expansion of low-carbon electricity. And that is a long way beyond, given that there is much more heat demand than electricity demand. To get an idea of the disparity, we re-generated the previous chart with all the lines plotted against the same axis.

Biomass Heat: The Seasonal Solar Storage Technology

We need several orders of magnitude more wind and solar electricity than we are currently generating if we want to electrify our heat supplies.

The Levy Control Framework, which covers the costs of the main subsidies for low-carbon electricity, is already projected to cost around £11bn, to meet around 30% of our existing electricity demand in 2020/21. For political or resource reasons, some of the cheaper contributors, such as landfill gas, sewage gas and onshore wind, are likely to be constrained in any future expansion, so electrification of heat would rely more heavily on more expensive low-carbon technologies.

The government might hope to employ various techniques to mitigate the disparities between heat demand and the production of low-carbon electricity. We tried to model an optimistic scenario that assumed they were able to deploy these techniques at scale and at a manageable cost, for weather conditions matching 2016.

  1. The easiest way to narrow the gap is not to aim to electrify all the heat. Let’s say that we aim to electrify only one-third of the heat. This is less than envisaged in 2027 in the core scenario for the existing Heat Strategy.
  2. On the basis that all the electric heat will be supplied by heat pumps, the electricity required to produce the heat is only a fraction of the total heat demand.
  3. Nuclear output has a positive correlation with heat demand, so let’s assume that we also increase our nuclear-generating capacity as well as our renewable generating capacity. That is beyond the replacement of our existing ageing fleet. Hinkley Point C, which is expected to cost consumers £30bn and enter production sometime around 2030, will replace around one-third of the existing capacity.[v] But we have assumed for this model that the political, economic and deployment challenges of doubling our nuclear capacity (replacing the existing capacity, plus adding the same again to supply the heat market) by 2027 can be overcome.
  4. Storage can help match some of the more predictable imbalances, particularly the short-term ones. We assume for the purposes of this model that the government chooses to provide enough subsidies to deliver the optimal amount of storage. There are also round-trip losses from storing electricity (i.e. you don’t get as much out as you put in). Our model allows for 70% round-trip efficiency.

On this basis, we plotted the hourly surplus or deficit between the electricity output and the demand for electricity for heating, assuming that:

  1. we installed as much extra onshore wind, offshore wind, solar and nuclear for supplying electrified heating, as was generated in 2016, following the production patterns of those technologies in 2016,
  2. the heat demand matched that of 2016, and was supplied entirely with heat pumps at ASHP COPs for the temperatures in each hourly period, and
  3. 30 GWh of electricity storage was installed (i.e. there is only a surplus if output has exceeded demand so much that the storage is full, and conversely there is only a deficit if demand has exceeded output so much that the storage is exhausted).

 Biomass Heat: The Seasonal Solar Storage Technology

Despite all of the very optimistic assumptions, there are winter periods of sustained cold weather with light winds and low insolation, when spinning reserve would be required to meet almost all of the electrified heat demand. We would need around 28 GW of fossil-fired spinning reserve – many times more than is currently available and already required for conventional electricity demand. This would require new installations of fossil-fired generating capacity. Unlike the utilisation of existing power stations, new spinning reserve is expensive, because the capital cost has to be recovered from the limited output periods. It also highlights that electrified heating would increase, not reduce carbon outputs for the coldest periods, even under the most benign set of assumptions.

There is also a large summer excess of low-carbon electricity, beyond that required to meet our heat requirements. This cannot simply be spilled to conventional electricity consumers because the amount of low-carbon electricity produced in summer from the capacity that is already planned and budgeted will exceed conventional electricity demand for large parts of the summer, even before we allow for the extra low-carbon generating capacity required for heat electrification. The value of most of this heat-purposed low-carbon electricity will be negative (i.e. producers will have to pay to put it into the grid) for most of the summer. This will significantly increase the cost of the subsidy required to persuade people to build systems for this purpose.

Energy storage for seasonal variations in heat demand

Let’s be frank. Even the one-third-electrification scenario is not feasible:

Are we prepared to spend something of the order of our annual defence budget on a strategy to convert one-third of our heat to decarbonised electricity?

The problem is the scale of the seasonal swings in demand for heat compared to the more manageable seasonal variation in conventional demand for electricity.

What we need is an affordable technology for storing renewable energy in summer and releasing it in winter during periods of high heat demand.

Biomass heat is that technology.

Biomass solar storage

Biomass takes solar energy and converts it into a stable fuel that can be stored and converted with simple, affordable technology.

It is as easy to imagine the roll-out of this technology at large scale as it is difficult to imagine the electrification scenario described above.

Pellet boilers

1 million rural properties with 10-tonne pellet stores would hold enough fuel to last most of them for around a year. The production and distribution of the fuel could be spread evenly across the year but would be available on demand.

This would provide 50 TWh of energy storage, at a cost of approximately £100/MWh of storage. And it would support around 20 GW of heat production during periods of peak heat demand.

Pellet stoves

10 million urban/suburban properties with pellet stoves could hold a pallet of pellets in the garage, or for urban dwellers, collect a few bags at a time from their local retailer. The retail chain could hold stock in conventional storage facilities.

The pellet stove would reduce the load on the primary heat source for the property during periods of peak heat demand, whether that heat source was a heat pump, a gas boiler (potentially using biomethane) or some other technology. This would significantly reduce the strain on these technologies when they struggle most to meet demand.

A simple space-heating stove is a low-cost appliance, which makes it ideal for the purpose of peak-lopping the heat demand. The use of pellets enables widespread deployment with manageable storage space requirements and the assurance that only high-quality fuel with low emissions can be used in the appliance.

This would support another 65 TWh of heating fuel ready to be released as required, up to around 50 GW during periods of peak heat demand. No investment would be required in specialist energy storage equipment.

Industrial biomass boilers

Larger heat users (primarily industrial) with more continuous heat demand would not need the benefits of densification to smooth out their seasonal demand and could take advantage of the lower fuel costs of wood chip and energy crops where available, making the most of the local biomass resource or potential.

The Forestry Commission’s Woodfuel Resource project ( estimates that the amount of woodfuel that could be available is a little less than 8 million oven-dry tonnes a year, equating to around 40TWh.

A 2015 report by the Energy Technologies Institute on Bioenergy: Enabling UK Biomass estimated the potential for energy crops in the UK at around 55 to 85 TWh annually in the 2030s, using 2.7 – 6.5% of UK agricultural land. These crops could be used to produce heat, electricity or transport fuels. The report does not predict what proportion would be available for heat.

Why not?

This reality has been perfectly apparent to most governments in developed nations for years. The following are the contributions from the various renewable-energy technologies across all uses in Europe in 2015.[vi]


Final energy (ktoe)

Solid biomass for heating




Onshore wind


Solid biomass for power


Heat pumps




Biogas heat & power


Solar thermal


Offshore wind


Geothermal heat & power


Bioliquids heat & power


Perennial energy crops


Tidal, wave and ocean


Biomass heat provides more than twice as much energy as hydro-electricity, and more than all the other forms of renewable heat and electricity combined. More than eight times as much heat as electricity is generated from biomass. And nearly nine times as much heat is generated from biomass as from heat pumps.

The UK’s plans differ significantly from the practice in countries that have successfully decarbonised their heat supplies. Biomass is expected to make almost no contribution in the core scenario followed by the current UK Heat Strategy.[vii] A number of reasons are given for British exceptionalism:


It is alleged that there is not enough resource of sustainable biomass for any significant quantity to be available for heat.

I addressed this issue in my last blog post for the WHA.

To put it into the context of this post, all of the wood pellets required for the scenario I described above (1 million pellet boilers and 10 million pellet stoves, requiring by my estimate around 23 million tonnes of pellets annually) could be produced from around 20% of the decline in industrial wood demand in the USA between 2005 and 2010. In other words, all we need is one-fifth of the wood for which there is no longer demand in one other country.

If the resource constraint were genuine, the ability of solid biomass to act as an affordable, seasonal store of solar energy means that, if it has to be prioritised, it should be prioritised for the use that is most difficult to decarbonise by other means because of the massive variations in demand: heat.


Some pressure groups dislike biomass ostensibly because they believe that it contributes to deforestation and the loss of habitat. They appear to be unable to differentiate between anecdote (for which the examples that they cite are no doubt genuine) and statistics (which demonstrate that they are wrong in the generality, regardless of the exceptions that they identify). If we have to stop doing things because some people breach the rules, we will have to stop all forms of human activity.

Air quality

There has been recent attention to the contribution of wood stoves and boilers to the poor air quality in some of our cities.

Biomass’s contribution has been exaggerated.[viii] But to the extent that there is a problem, it relates largely to two factors:

  1. Inadequate combustion conditions, such as may be created in open fires or old stoves, and
  2. Poor-quality fuel, such as recovered wood, wet wood or non-wood combustibles (e.g. plastics).

There is no practical way that people can be prevented from using their open fires and old stoves, nor that the fuel they use can be controlled. The best way to tackle this problem would be to encourage the replacement of these installations with a new, good-quality appliance that can only accept high-quality fuel.

That is exactly what a roll-out of pellet stoves would achieve. Pellet stoves can only burn wood pellets, and the only form of wood pellet widely available in bagged form is the highest-grade: ENplus A1.

The mass replacement of open fires and old stoves with well-installed new pellet stoves should have a beneficial effect on air quality.

User experience

There have been some unfortunate problems with biomass heat installations.

This is largely a function of the way that the market was over-stimulated by the first phase of the RHI, creating a gold rush that attracted too many cowboys.

These problems could be addressed with the benefit of hindsight from phase 1 of the RHI.


Biomass heat is one of the cheaper forms of renewable energy. Its opponents try to twist logic to argue that it is nevertheless not good value in one of two ways:

  1. It has no role in our future heat supplies, so any money spent on it is down the drain, even if it is currently cheaper than the “strategic” technologies.
  2. Small biomass is more expensive than larger biomass, so any support above the level needed to deliver large biomass is effectively dead weight cost.

These arguments rely on such palpably bad logic that it should be obvious that their purpose is less to convince and more to deflect.

No. 1 is circular reasoning. If we accept this logic, we should never use whatever technology is most cost-effective now, if “experts” have decreed that a more expensive technology will be more significant in the future. Price signals are dead; economic evolution is replaced by central planning.

No. 2 is a false dichotomy. It relies on false assumptions about the scale of the resource and how it should be prioritised. Small and large biomass systems are not the only options. The true alternatives are all the renewable heat technologies. Small biomass heat is still cheaper than most of the other technologies.


The straightforward way to discover which are the economically optimal solutions to reduce our carbon footprint would be to apply a carbon price to all emissions from fossil fuels (and to equivalent factors that increase or decrease atmospheric concentrations of greenhouse gases).

There may not be the political will to implement such a measure, but imagining such a world provides a useful thought experiment to judge whether the alternative measures are achieving a similar effect. What combination of technologies would deliver our environmental objectives at a lower carbon price than is required by small biomass heat?

The deployment of biomass heat described above is a feasible and deliverable option using established technologies to radically reduce carbon emissions from heat production.

Once one considers the reality of the scale and seasonal variations in heat demand, the patterns of production of low-carbon electricity, and the challenges of long-term electricity storage, what alternative strategy could achieve similar results for similar costs, without relying on magic bullets that have not yet (after nearly 30 years of support for low-carbon electricity) demonstrated their commercial viability?

Note: The Board of Directors of the WHA will each month publish a blog on topics related to the biomass heating industry. The views expressed in this blog post are those of the author(s) and do not necessarily reflect the views or policies of the WHA/REA. The article is a condensed version of an article published on Forever Fuels’ website.[ix] The arguments rely on detailed analysis carried out by Forever Fuels, which is explained in more detail in that article.

[vi] European Environment Agency, Renewable Energy in Europe 2016 – Recent growth and knock-on effects, Tables 2.6 and 2.7 (
[vii] Redpoint/Baringa, Modelling to support The Future of Heating: Meeting the Challenge (2013), pp.20, 24, 27, 29, 31 (