At the start of this year (end of Christmas holidays! time flies!) I did a little bit more work on the dynamic heat pump simulator but did not update this thread about it at the time. I did discuss some of the results from the new features in this thread here: What SCOP can you expect from a system that runs at 55C and 50C flow temperatures on the coldest days?
The latest version now includes:
- Ability to simulate space heating performance over a full year
- Interpolated COP model option based on the detailed Vaillant datasheets
- Carnot COP model option with heat output proportional DT’s between flow temperature and condensing temperature, outside temperature and evaporator temperature.
- Agile Octopus based tariff cost comparison.
- Option to block of hot water periods - though not the option to simulate hot water runs yet.
The main features that are still really needed are a realistic defrost model and hot water cylinder simulation.
Check out the latest version here: https://openenergymonitor.org/tools/dynamic_heatpump_v1
Continuous vs intermittent heating
I thought I’d share an interesting result on the topic of continuous vs. intermittent heating. It’s so difficult to test this in the real world, given how much outside conditions change on a day-to-day or week-to-week basis and how the thermal mass of a building will always be at varying levels of “thermal charge.” Apart from running such scenarios in a controlled environment, such as the Salford Energy House, I think simulation tools are probably our best option to explore this question. I think the heat pump industry would benefit from much more sophisticated simulation tools to explore these questions.
Yes the simulation suggests you can save electric with intermittent heating and a lower SPF - but it’s more expensive on Agile Octopus:
In this example I created a baseline scenario with a 20C all the time schedule and then compared it to a schedule that effectively turns the heat pump off over night between 10pm and 8am.
- The SPF drops from 4.55 to 4.25.
- Electricity consumption drops by 13%
- This fixed tariff saving is reversed if your on octopus agile where it becomes 1% more expensive to run intermittent compared to running continuous.
- Indoor temperatures drop as low as 13.7C towards the end of the night in this simulation so there’s a comfort impact there.
- Another way of looking at it is that you can bask in 20C indoor temperatures all day for only 13% more cost on a fixed tariff or no extra cost on agile.
Another interesting variation is a schedule for folk that are out of the house all day until the evening, the simulation suggests that letting the heat pump turn off between 10pm and 2am and then come on at a lower level during the day before boosting up to 20C after 3pm can save almost 17% in terms of electric consumption and fixed tariff costs. The SPF drops from 4.55 to 4.08.
On Agile again however this is more expensive still (lower comfort is 5% more expensive than continuous heating this time), which we would expect given that consumption is more focused on peak time hours.
This is an interesting result as it contradicts the simple rule of thumb that continuous will always be better than intermittent heating for heat pumps. It looks like there could be a benefit for folk on fixed tariffs even with the drop in SPF.
In practice however the interaction with time of use tariffs, solar and batteries and operation that aims at minimizing grid load at peak times is likely to favor continuous heating or tariff and solar aligned boosting of indoor temperature even if electricity consumption is slightly higher.