Renewable Heat

There are a number of different approaches to providing space and water heating with a renewable supply. Perhaps the two most well known renewable heat technologies are heat pumps powered by renewable electricity and biomass heat.

The key limitation with biomass heat is of course land use. There is a limited amount of land to share between food production, wild spaces, carbon sequestration and biomass energy. Growing biomass for energy on a large scale reduces the amount of land available for these other demands.

Heat pumps with the bulk of their power from renewable electricity: primarily wind and solar provides the opportunity to reduce the amount of land required for biomass energy significantly, however there is still a need for a backup electricity supply when the wind is not blowing. The hourly energy model suggests that around 75% of a heat pumps electricity demand can be supplied directly from a wind and solar supply mix (at equal capacities of each) with 25% needing to be sourced from a backup supply. As explored in the appendix below, this backup requirement stretches several orders of magnitude beyond the capacity of today’s battery technologies and so another solution is required. ZeroCarbonBritain uses biomass and hydrogen produced from excess wind and solar to generate large quantities of methane gas which can then run gas turbines to generate the backup electricity required.

The following section explores 10 different renewable heat approaches from biomass only approaches to combinations of biomass, heat pumps, CHP and finally biomass as a backup for heat pumps running off primarily wind and solar electricity. It explores the land use implications of each approach.

A household heat demand of 6100 kWh/year for hot water and space heating is used, which is based on a just over 50% saving from better insulation and draft proofing. The land areas discussed below would be slightly more than double if building fabric energy efficiency remained at today’s levels.

Biomass only

High efficiency biomass boilers 8-14%

High efficiency biomass boilers have an efficiency that typically exceeds 90%, such as the Okofen Pellematic (Calorific efficiency of 98.7%), the Eco Angus Log batch boiler (92%), or Froling T4 (94.2%).

6100 kWh/year or 16.7 kWh per day of heat demand translates to a dry biomass demand of 16.7 kWh/d / 0.92 = 18.2 kWh/d dry biomass input at 92% efficiency.

A estimated further 10-20% is lost in production (appendix 1), drying the wood chip or pellet feed stock, chipping & pelletising. If we assume the lower bound of 10% (assuming a high degree of air-drying), 20.2 kWh/d of dry biomass input is required to provide 16.7 kWh/d of heat.

The following table with yield amounts given in the ZeroCarbonBritain land use methodology give the power density of different biomass energy sources:

odt/ha/yr kWh/ha/yr kWh/m2/yr W/m2 TWh/Mha
Productive Broadleaf Woodland 0.38 1793.6 0.18 0.02 1.8
Productive Coniferous Woodland 0.8 3776 0.38 0.04 3.8
Short Rotation Forestry (SRF) 2.77 13074.4 1.31 0.15 13.1
Short Rotation Coppice (SRC) 12 56640 5.66 0.65 56.6
Miscanthus 14.5 68440 6.84 0.78 68.4
Rotational Grasses 9.5 44840 4.48 0.51 44.8
Hemp 6.75 31860 3.19 0.36 31.9

odt: oven dried tonnes, Modt: Mega oven dried tonnes.
4.72 Twh/Modt, 17 MJ/kg, 4720 kWh/odt

Oven dried tonnes per hectare per year are converted here to power production, per unit area (Watts/m2) for easier comparison with David MacKay’s chart and analysis:

20.2 kWh/d is equivalent to 842 Watts continuous power. Wood chips and pellets can be produced from broadleaf woodland, coniferous woodland, short rotation forestry, short rotation coppice and miscanthus.

The highest yield is provided for by miscanthus 0.78 W/m2. Using miscanthus would require 842 Watts / 0.78 W/m2 = 1080 m2 per household.

If we divide the land area of the UK by 26 million households we arrive at 9231m per household and so direct biomass heat with miscanthus would use about 12% of UK land area.

Another way to picture the land area required per household might be to compare it with the size of football pitch at 7350 m2. One household would require 15% of a football field, or alternatively two times the area of a typical penalty box.

If short rotation coppice was used instead of miscanthus this would increase to just under 1300 m2 or 14% of UK land area.

If productive coniferous woodland was used we would need 21,050 m2/household or 2.3x the UK land area, increasing to 4.6x UK land area with productive broadleaf woodland. Given that a large degree of biomass heat today is from these slower growing sources it highlights the need to develop the higher yielding crops if we are to achieve a sustainable level of land use at scale.

ZeroCarbonBritain suggests that yields for SRC and miscanthus can be improved by 50% (see land-use methodology). With such an improvement SRC would provide 0.975 W/m2 and Miscanthus: 1.17 W/m2. Resulting in land area use of 9.4% and 7.7% respectively.

"Grass for gas" standard AD (26%)

An alternative approach is the production of biogas, injected into the gas grid from Anaerobic Digestion plants. Anaerobic digestion converts wet biomass in to biogas at an energy efficiency of 60%.

AD requires wet biomass such as rotational grasses. These have an energy content of about 0.51 W/m2. Converted to biogas the energy content is equivalent to 0.31 W/m2. Assuming no losses associated with transportation in the gas network and a gas boiler efficiency of 92% resulting in a gas demand of 18.2 kWh/d or 756W. 2440 m2 of rotational grasses are required, equivalent to 26% of UK land area if scaled up for all households.

"Grass for gas" with enhanced sabatier processing (16%)

Biogas from Anaerobic Digestion has a relatively high CO2 content. ZeroCarbonBritain suggests the addition of sabatier reactors to AD plants that can combine hydrogen generated from excess wind and solar electricity with CO2 from the AD biogas to create a larger quantity of methane.

2.0 kWh of biomass + 1.0 kWh of hydrogen = 2.0 kWh of synthetic methane

With sabatier enhanced anaerobic digestion, methane gas could be produced at a power production per unit area of 0.51W/m2 which would reduce the amount of land required to 1482m2 or 16%. Which is not far off our estimate for the land area requirement of short rotation coppice in biomass boilers above.

Heat pumps & Biomass backup

The next series of examples explore the addition of heat pumps starting with heat pumps powered solely from electricity generated from biomass and then adding in renewably electricity from wind and solar with biomass electricity used as a backup.

Heat pumps + standard AD (21% biomass only, 5.1% wind & solar backup)

We start here again with rotational grasses at a land efficiency of 0.51W/m2. 40% of the energy content is lost in conversion to biogas. Standard high efficiency electricity generating gas engines in AD plants have efficiencies of the order of 40% resulting in electricity at a land efficiency of 0.1224 W/m2. A heat pump then converts this to a heat efficiency of 0.3672 W/m2. Our heat demand was 16.7 kWh/d or 696 Watts.

0.51 W/m2 x 0.6 x 0.4 x 3.0 = 0.3672 W/m2 696 Watts / 0.3672 W/m2 = 1895m2, or 21% of UK land area

If standard AD is only used as a backup electricity source covering 25% of the electricity demand the land area requirement for the biomass backup would be:

75% wind and solar, 25% biomass backup = 1895m2 x 0.25 = 474 m2 (5.1%)

Heat pumps + standard AD + CHP heat recovery (14% biomass only, 3.6% wind & solar backup)

Very similar to our standard AD only example above, reciprocating gas engines with an electrical efficiency of 40% also support heat recovery of up 50%.

This utilises an additional 0.153 W/m2 above 0.3672 W/m2 achieved with power generation only. Reaching 0.5202 W/m2

100% biomass: 696 Watts / 0.52W/m2 = 1338 m2 (14%)

75% wind and solar, 25% biomass backup = 1338m2 x 0.25 = 335 m2 (3.6%)

Heat pumps + sabatier enhanced AD (10% biomass only, 2.5% wind & solar backup)

We start here again with rotational grasses at a land efficiency of 0.51W/m2. 0.51W/m2 of higher quality methane is produced. This can then be burnt at up to 60% efficiency in large CCGT gas turbines. Although 50% may be more representative and is the efficiency used in ZeroCarbonBritain. The electricity produced has a land efficiency of 0.255 W/m2

A heat pump then converts this to a heat efficiency of 0.765 W/m2. Our heat demand was 16.7 kWh/d or 696 Watts.

696 Watts / 0.3672 W/m2 = 910m2, or 10% of UK land area

75% wind and solar, 25% biomass backup = 910m2 x 0.25 = 228 m2 (2.5%)

Heat pumps + CHP Wood gasifiers (5.1% to 9.2% biomass only, 1.3% to 2.3% Wind & Solar backup)

An alternative to the use of wet biomass and anaerobic digestion is to use woody biomass such as short rotation coppice and miscanthus with a gasifier. Typical efficiencies for gasifiers are less than what can be achieved with biogas. Electrical efficiencies of 20-30% and Heat efficiencies of 50-60% are possible. EL: 28%, HEAT: 56%
Entrade E3, Electrical efficiency 25%, Thermal efficiency 60%

Today’s yields

Miscanthus 0.78 W/m2
Heat pump heat: 0.78×0.9×0.3×3=0.6318
Heat recovery: 0.78×0.9×0.5=0.351
Total: 0.9828W/m2, 708m2 (7.7%)

75% wind and solar, 25% biomass backup = 708m2 x 0.25 = 177 m2 (1.9%)

SRC 0.65 W/m2
Heat pump heat: 0.65×0.9×0.3×3=0.5265
Heat recovery: 0.65×0.9×0.5=0.2925
Total: 0.82W/m2, 850m2 (9.2%)

75% wind and solar, 25% biomass backup = 850m2 x 0.25 = 213 m2 (2.3%)

Higher future yields

Miscanthus 1.17 W/m2
Heat pump heat: 1.17×0.9×0.3×3=0.9477
Heat recovery: 1.17×0.9×0.5=0.5265
Total: 1.4742W/m2, 472m2 (5.1%)

75% wind and solar, 25% biomass backup = 472m2 x 0.25 = 118 m2 (1.3%)

SRC 0.975 W/m2
Heat pump heat: 0.975x0.9×0.3×3=0.78975 
Heat recovery: 0.975×0.9×0.5=0.43875
Total: 1.23W/m2, 567m2 (6.1%)

75% wind and solar, 25% biomass backup = 567m2 x 0.25 = 141 m2 (1.5%)

Heat pumps with direct biomass boiler backup (1.9% to 3.5%)

An alternative to supplying backup electricity for heat pumps generated from biomass is to use a combination of a heat pump and a biomass boiler usually known as bivalent heat pump heating. Bivalent systems are usually installed where the heat pump cannot cover all heat demand or where the COP of typically an air-source heat pump drops below a particular level due to extreme cold ambient conditions. In these conditions the heat pump system is either supplemented by a backup heating system or the system switches over completely. This function could be extended to use the backup heating system where wind and solar supply low.

If we assume as above that 75% of the heat pumps electricity demand is provided by wind and solar electricity and that the backup requirement remains at 25% of demand this means that we need:

16.7 kWh/d head demand x 0.25 = 4.2 kWh/d of backup heat

Provided for in a 92% efficient biomass boiler this results in a dry biomass demand of:

4.2 kWh/d backup heat / 0.92 = 4.5 kWh/d dry biomass backup

Assuming a further energy loss of 10% in drying and processing as in our biomass boiler only example above results in an input biomass requirement of:

4.5 kWh/d dry wood chip/pellets / 0.9 = 5.0 kWh/d (208 Watts) wet woody biomass
Crop Yield Land area per household
Miscanthus 0.78 W/m2 267 m2 (2.9%)
Miscanthus (150%) 1.17 W/m2 178 m2 (1.9%)
SRC 0.650 W/m2 320 m2 (3.5%)
SRC (150%) 0.975 W/m2 213 m2 (2.3%)

These estimates suggest that using direct biomass boilers for backup in this way might use 35% more land area than the minimum land area that might be achievable with a wood gasifier CHP system running at high efficiency.

Another way of looking at it could be to assess both methods in comparison with a 100% biomass baseline, in this case the bivalent system would reduce land area requirement by 75% and the CHP wood gasifier system by 84% providing an additional 9% saving.

More detailed modelling would be required to assess the impact of low outside temperatures on heat pump COP with an assessment of benefits of reducing heat pump requirements both at times of low wind and solar output and low temperatures.


1. Production of wood chips and wood pellets

Short rotation coppice yields are currently around 0.65 W/m2 this may increase in future as SRC is breeded for better energy crop performance. ZeroCarbonBritain models a 50% improvement as part of its model, increasing present yields to 0.975 W/m2.

Energy lost in pellet and wood chip production:

25 tonnes wet wood chip, dried results in 18.75 tonnes output. 2.25 tonnes ussed for next cycle, producing 16.5 tonnes of dry wood chip. An efficiency of 12%

Drying greatly improves final burn efficiency: 35-40%MC (542 kWh/m3), 15-20%MC (833 kWh/m3) (borders_woodfuel_drying_presentation.pdf)

Drying makes up 70% of process energy (BioRes_05_4_2374_Pirraglia_GS_Techno_Econ_Anal_Wood_Pellets_US_Prodn_1108.pdf)

Overall energy lost could be as high as 20% prior to use, but could also perhaps be significantly less with higher efficiency drying methods such as air drying and solar heated air drying as discussed here, which could perhaps approach a 10% processing loss:

2. Calculating the backup requirement of heatpump demand

The heatpump backup requirement of 25% was estimated using the ZeroCarbonBritain based 10 year hourly energy model that we have developed here: 8. Full Household Energy Model. To replicate the result follow these steps:

First simplify the default model to remove the direct synthetic liquid and gas production processes, this makes comparison across different scenario's easier.

  1. Starting with default model
  2. Under 'Synthetic liquid fuel' and 'Synthetic methane' untick supply unmet demand from less efficient processes.
  3. Increase gas store to 5000 kWh and starting level to 2500 kWh
  4. Increase wind supply to 1.9kW and solar to 1.9kW so that methane gas store ends at a similar level to its starting level. Ending higher than the start means that there is more renewable generation than needed. Ending lower means that there is unsufficient renewable generation which may mean an empty gas store in further years.

Total supply: 26.6 kWh, Total demand: 17.0 kWh, Primary Energy Factor: 1.57, Excess: 0.4 kWh/d, Total CCGT Output: 2.2 kWh/d.

  1. Drop heatpump demand by setting heatpump fraction to 0 under the "Heating System" section.
  2. Reduce wind to 1.22 kW and solar to 1.22 Kw, so that methane store start and end is similar.
  3. Change store size to 3000 kWh and start 1500 kW as the larger store is not needed.

Total supply: 17.1 kWh/d, Total demand: 11.4 kWh/d, Primary Energy Factor: 1.50, Excess: 0.0 kWh/d. Total CCGT Output: 0.8 kWh/d

Total backup associated with heatpump is 2.2 kWh/d – 0.8 kWh/d = 1.4 kWh/d
Total backup as proportion of heatpump demand is 1.4 kWh/d / 5.6 kWh/d = 25%

When heatpump demand is considered in isolation of other demands such as lights, applicances and cooking and electric transport demand, the degree of backup increases to around 30%, suggesting that some of the additional supply required for these demands can be used at certain times by the heatpump resulting in a higher degree of matching.

3. Calculating lithium ion battery capacity required to meet heatpump backup requirement

It is possible to use the 10 year hourly energy model to estimate the size of a lithium ion battery required to backup heatpump demand without using biomass at all.

To do this we start by simplifying the model by removing air transport demand and synthetic methane production for backup supply in order to make the model an electric only model.

The biomass, synthetic methane scenario uses a significant amount of wind and solar over-supply in order to produce hydrogen from exess wind and solar that can then be combined with biogas in order to create a larger quantity of methane gas. In order to provide a fair comparison the same level of over-supply is used in both our hypothetical battery scenario and the methane backup scenario.

Steps to calculate battery backup size:

  1. Remove air miles
  2. Remove synthetic liquid production
  3. Adjust wind and solar so that excess generation is near zero

Result: primary energy factor 1.54 (AD + sabatier process uses 54% of additional wind and solar supply to generate backup for lights, appliances & cooking, electric cars and heatpumps). We use this same over-supply extent for our battery capacity estimate.

  1. Remove sabatier process and AD.
  2. Remove backup gas turbines
  3. Increase wind and solar so that primary energy factor is 1.54

Result wind: 2.22, solar: 2.22, matching: 89%

  1. Adjust battery so that 100% supply demand matching is required.

Battery capacity to cover all backup demand including heatpumps: 800 kWh

  1. Remove heatpumps
  2. Adjust wind and solar production so that primary energy factor is 1.54. Result onshore wind: 1.15 kW and Solar PV 1.15 kW.

  3. Adjust battery so that 100% supply demand matching is required.

Battery capacity to cover all backup demand without heatpumps: 110 kWh

Conclusion: 690 kWh battery required to cover heatpump demand.

The amount of battery backup required to cover heatpump demand is between 50 and 100 times the typical capacities of today's domestic battery storage. Suggesting that significantly lower battery costs, embodied energy levels and perhaps higher energy densities would be required to make battery storage a viable option for covering winter backup demand's of heatpumps.

Lithium battery storage is better suited for hourly and daily storage demands and grid stabilisation, small stores are likely to play a very useful role when used with heatpumps by making it possible to move heatpump demand away from times of otherwise peak grid electricity demand, reducing grid and backup gas generation capacity requirements. Our hourly energy model does not yet model this role for battery storage very well and requires further development.

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