How do you model heat pumps to determine the optimal energy generation & storage system?

Wow! That’s a long title, and I just couldn’t work out how to make it any smaller.

However, if we are being told that a large percentage of existing & new homes / businesses will be moving to some form of heat pump over the coming decade, how can we help them (and the people financing the investment) clarify that they have opted for the right overall system design that makes the best investment and low carbon case – this means the right combination of renewables & energy storage alongside their electrified heating system.

The key is to start from the heating basics.

Heat Demand Basics

Heating demand, although changing with the external and internal temperature differential, should be your baseline energy calculation (along with your power consumption).

The best way to achieve this is to be able to access your half-hourly (or hourly) heating demand from your energy supplier.

However, very few homes & / or business do – certainly I do not have this for my own home. This is because most gas & oil supplies are based on quite old metering technology that has no form of communication attached.

If you do not have this data (which you probably won’t), then you can look to use your monthly / quarterly heating demand numbers, which you can get by either speaking with your energy supplier or by recording the values on your meter yourself.

Again, most energy suppliers only take readings every 6 months and there are a few of us who may be willing to go and get the meter reading at the same time each month.

This means that most of the time most people are left an annual consumption value and feeling that modelling their heat demand is a little too hard for them as opposed to electricity where data seems to be more easily available.

This means that in the absence of real data, we need to build a picture of what we expect the heating demand to be throughout the year.

In brief, the heating energy requirement at any period can be calculated as follows:

(SHLprop x tdiff) / Effbolier

Where SHLprop is the specific heat load of the property (determined by the insulation levels and air tightness), tdiff is the temperature differential between your desired internal temperature and the external temperature and Effboiler is the efficiency of your existing boiler.

I am kind of simplifying things here, but it helps to create a foundation of the maths.

We then sum this result for each period over the year to get the total heating demand required.

Now, the problem is that you or me trying to do this on our own is going to lead to us giving up before we start. And the problem is that we really do need some sort of granularity if we are going to properly model the impacts of converting to a heat pump & overlaying this with renewables and / or energy storage.

The good news is that in GridMAP we can make this easy for you by either:

Allowing you to upload an hourly or half hourly file of projected heating data, or
Allowing you to enter an annual heat demand (in kWh) and then we’ll automatically create an annual half hourly heating profile for you (using your project location & air temperature databases.)

The image below shows the actual power & heating energy required for an example site in our GridMAP software. You can see how the winter months have heating demand in orange driven by the target temperature and expected overall heat requirement.

Once you have your heating demand data then you need to apply the theory of a heat pumps to calculate the projected electrical demand impact on your existing Grid import.

Most projects, I believe, will be air source heat pumps (ASHP) as most people will not have the available land nearby for ground source (GSHP), so I’m going to focus on ASHP for the moment.

Air Source Heat Pump Fundamentals

ASHPs work by taking the heat from the external air and compressing it (using electricity) to produce an increase in temperature that can be used for internal heating & hot water (although generally for heating as they work best at lower temperatures.)

This increase in temperature can be seen as a conversion of energy. In brief, a specified unit of electricity is used to create heat energy. The ratio of the heat energy output to the electrical energy input is referred to as the Coefficient of Performance (CoP) and is the critical determinant of how well a heat pump (HP) will work.

Typically, many in the industry will refer to a CoP level between 3 and 4, although there would seem to still be debate as to the actual values seen in real live installations – however, that is for another discussion.

It should be noted that ASHPs will have a varying CoP throughout the year because when the air is really cold it will take more electrical energy to create the thermal output and vice versa. However, for modelling purposes it is normal to use a fixed value that is the average for the year.

So, with this information we can then create an expected electrical demand profile for heating by taking each period’s projected heating demand and dividing it by the CoP.

The heat pump electrical profile that is produced can then be added to the existing power demand profile to create a projected full electrical demand profile for both power & heating (plus hot water).

Heat Pump Savings

Now here is the key thing.

When you convert to a heat pump, there are 2 potential changes to your financial & environmental impacts:

1. Carbon Impacts of Heat Pumps

Firstly, there is a carbon impact. If you have a CoP of 3, this means that you are importing 1 unit of electricity from the Grid (prior to considering generation / storage) and saving 3 units of fossil fuel import (generally gas).

Currently the electrical Grid in the UK has an annual average carbon intensity of approximately 0.2 kgs / kWh. Interestingly, this is almost the same as natural gas which we will call the sane value for the ease of calculation.

That means that our net carbon benefit with a CoP of 3 is as follows:

Net Carbon (kgs per kWh electric) = (1 x 0.2) – (3 x 0.2) = -2 x 0.2 = -0.4 kgs CO2 per unit of electricity

This is a significant change from 10 years ago when the electrical Grid had an intensity of 0.5kgs / kWh when the carbon savings would have been lower by a factor of 4 and almost equal to zero i.e. no carbon benefit.

This means that the carbon value associated with installing heat pumps has changed for the better significantly over the last decade.

2. Tariff Benefits of Heat Pumps

The same sort of equation can be undertaken for the tariff impacts.

However, it is not so clear as to whether the impact will be positive or negative given the big changes in gas prices and the possible implications for electricity prices.

As an example, imagine your gas price is 4.5p / kWh and your electricity is 18.5p / kWh (my current rates with Good Energy). Using the CoP of 3, we can determine that:

Net Tariff Impact (p / kWh) = (1 x 18.5) – (3 x 4.5) = 4.5p cost increase per unit of electricity

With these examples (and they are just examples), you are saving on carbon, yet paying more for your energy.

This could be translated as a cost of 4.5p / 0.4kgs of CO2 or a cost per tonne of £112.5. Interesting numbers to debate J

It should be noted that in the UK there is currently a generation tariff for heat pumps that will annul this tariff loss and provide additional income to incentivise their take-up, although this is being removed in early 2022 in favour of grants.

Putting it all Together

So, what this means is that once we are able to create an appropriate heat demand profile, we are then in a position to undertake an appropriate heat pump energy, carbon & financial appraisal.

With this, we can then start to apply an iterative process of assessing different solar & / or energy storage systems to determine which will mitigate the greatest Grid import (as electrical demand will increase from current) and in such a way as to reduce carbon and make the investment wash its face.

Heat & GridMAP

This is where GridMAP comes in and is starting to be used to enable clients to build projects with the analysis to understand how converting to a heat pump will impact their energy, carbon & financial costs.

By enabling users to enter multiple storage & generation sizes, the different permutations can be assessed alongside each other to determine which is the most appropriate.

With each, there are critical questions:

How should we optimise storage to maximise the investment case with a heat pump?
How does solar help and to what extent?
What is the net carbon that we still need to account for i.e. that which the storage & generation system cannot mitigate against and must be imported from the Grid?

Including heat pump analysis in GridMAP is a critical component if we are to transition rapidly to a system that is electrified and where heat is predominantly provided by heat pumps or equivalent technologies.

There is so much more to discuss but I’ll leave that for another blog.

I hope you’ve found this one useful.

Please get in touch with any questions you may have.

Fraser Durham

Commercial Director

Argand Solutions

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