Heatpumps in the ZeroCarbonBritain model by the Centre for Alternative Technology.

Following on from last blog post on heat pump monitoring I will try in this post to give a bit of context on heat pumps. Heat pumps are interesting because they provide a way to make efficient use of renewable electricity for heating. They are used in many zero carbon energy scenario's including the ZeroCarbonBritain scenario from the Centre for Alternative Technology which is one of the most comprehensive UK based 100% renewable energy scenarios created.

This sankey diagram from the ZeroCarbonBritain report gives an overview of the energy flows in their scenario, with heat pumps providing the bulk of the heating and hot water demand:

Heat pumps provide 66% of final space and water heating demand, providing 155 TWh of heat from 50 TWh of electricity – an average coefficient of performance of 3.1.
ZeroCarbonBritain also include 30 TWh of direct electric heating. The total input electricity for heating is 80 TWh and total heat delivered 185 TWh which together is an average COP of 2.3.

This 80 TWh of electricity demand is around 20% of the final direct electricity demand or 11% of the total renewable electricity generated as 47% of the electricity generated is used during times of excess production for export and synthetic fuel production, plus backup power to gas.

By using heat pumps for heating ZCB reduces the amount of renewable energy that would otherwise be needed to provide our energy needs.

The total domestic space heating demand projected is 114.4 Twh, divided by the projected 2030 number of households of 30.1 million this equates to 3800 kWh/year per household. This is the space heating demand after fairly ambitious space heating energy savings (62% reduction), made by improving building fabric with insulation, draught proofing and better heating control.

The total domestic water heating demand is projected to be 76 TWh, or 2525 kWh/year per household. ZeroCarbonBritain has 33% of this coming from solar hot water. The hot water demand provided for by heat pumps and direct electric heating is therefore 1694 kWh.

If the use of direct electric heating and heatpumps are evenly shared between all buildings achieving an average COP of 2.3 as above then our total delivered heat requirement of 3800 kWh + 1694 kWh = 5494 kWh becomes an input electricity requirement of 5495 / 2.3 = 2389 kWh/year per household or 6.5 kWh per day per household.

The average household in 2030 is assumed to have the same average occupancy as today of 2.34.

Our per person target would therefore become 2.8 kWh/d/pp of input electricity for space and water heating.

Space heating only Water heating Space heating and water heating Lighting, cooking and appliances
Heat demand 3800 kWh 1694 kWh 5494 kWh

Input electric 1542 kWh
@ 2.5 COP
847 kWh
@ 2.0 COP
2389 kWh
@ 2.3 COP
1636 kWh
Daily input electric 4.2 kWh/d 2.3 kWh/d 6.5 kWh/d 4.5 kWh/d
Per person 1.8 kWh/d/pp 1.0 kWh/d/pp 2.8 kWh/d/pp 1.9 kWh/d/pp

- 346 kWh cooking after 40% reduction, 1290 kWh lighting and appliances after 61% reduction

These figures provide a useful guide to the level of electricity consumption per household and per person that a scenario like ZeroCarbonBritain is aiming for and highlights the role of building fabric efficiency in reducing the heating requirements to start with.

The average COP achieved taking into account a mix of heat pump heating at a COP of 3.1 and direct electric is 2.3. If it where achievable to increase the combined COP to 3.0 by reducing the amount of direct electric heating this could either be used to reduce the input electric demand by 23% reducing the total amount of renewable energy that would need to be installed or it could alternatively be used perhaps to reduce the amount of building fabric improvement required to 45% rather than a 60% reduction.

The exact target will always be something that is in flux and will be a balance point between all the demands for renewable electricity and the degree to which we deploy renewable energy but the above figures give an idea for what we might want to try aiming for when designing homes that are ready for a zero carbon renewable energy supply.

Interestingly the total electricity demand including lighting, appliances, cooking, space heating and water heating is 11 kWh/d per household in the ZeroCarbonBritain scenario which is only 3.3% above the 2007 electricity only baseline. The additional demand from the electrification of heating is negated by the increase in efficiency in lighting, cooking and appliances.

The 60% energy saving target for space heating taken into account above is not actually state of the art. 3800 kWh/year in an typical uk home with a floor area of 85m2 is 45 kWh/m2/year. Passivhaus new build achieves a space heating energy demand of 15 kWh/m2/year and retrofit 25 kWh/m2/year.

A passivhaus retrofit with floor area of 85m2 would require 2125 kWh/year of heat, delivered at a COP of 3.0 would require 708 kWh of input electric, or 850 kWh if a COP of 2.5 is achieved, which may be more likely as a larger portion of the heat supplied will be at a higher temperature for hot water heating rather than space heating. 

Average home today

Lighting, Appliances and cooking
electric input:
3885 kWh, 10.6 kWh/d

Space heating gas input:
12500 kWh, 34 kWh/d

Water heating gas input:
2118 kWh, 5.8 kWh/d

Total: 18,503 kWh provided by a mix of gas and 2007 grid average electric = ~ 5 tonnes of CO2//year
Average ZeroCarbonBritain home

Lighting, Appliances and cooking
electric input:
1636 kWh, 4.5 kWh/d

Space heating electric input:
1542 kWh, 4.2 kWh/d

Water heating electric input:
847 kWh, 2.3 kWh/d

Total: 4025 kWh provided by renewable electric
at zero carbon

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