Battery or thermal storage for HP timeshifting

[CharlieB]

Hey folks. This is just me thinking out loud really, but I'm fairly sure others have done similar thinking.

We've just installed a 14kW heat pump which is working well. I'm keen to enable it to run only during non-peak times and have the potential to provide DSM services or respond to live tariff changes. I'm thinking generically, though, not just about our situation.

Battery A 3-4 kW battery with perhaps 9-12kWh would enable the heat pump to run entirely off battery for 3-4 hours.
But efficiency is only ~75%, lifetime is uncertain and I think lithium etc should all be in cars.

Big thermal store The point would be to seperate the operation of the heat pump from the operation of the heating, either partially or completely. So for the same 3-4 hours you'd need maybe 40-60kWh of thermal storage. A bit more to take into account losses. (And maybe a lot more if there's some thermodynamics going on that I've not thought about.) Some thoughts:

A. What volume of water would equate to 60kWh at a Delta-T of say 50 degrees c? More or less volume than the bunded kerosene tank that's now defunct. (Google suggests 14kWh in a 300 litre tank so hopefully easily less.)

B. Losses. Thermal store would almost certainly be outside and would be kept at radiator temp fairly constantly. So you'd have thermal losses 24/7 rather than just on the battery charge/discharge. All the same, I think without having calculated it that overall losses would be lower than the battery. ??

C. Heating performanmce. As thermal store cooled, because heating is on and HP isn't during peaks, I suppose radiators temps would go down. I can't intuit by how much though, and whether it would make much difference.

D. I've intentionally not asked about cost for now. Just trying to work out what I'm failing to think about.

E. Any other thoughts from you lot more than welcome!

Charlie

[nowty]

I run up to 3 heatpumps off my battery bank all day, no problem with the help of a couple of storage heaters for the coldest months. Storage heaters give more denser energy storage than water.

My last bill, I tend to stay away from peak rate.



My hot water tank is outside, only loses about 2 degrees per day in the coldest weather, just needs adequate insulation.

[Moxi]

Hi Nowty

Is your outside HWC installed in a brick lean to or something else ?

Moxi

[nowty]

Hi Nowty

Is your outside HWC installed in a brick lean to or something else ?

Moxi

Sort of, the general area is under a car port.

[GarethC]

Round trip efficiency of a battery is quite a lot better than 75% I would think?

[Krill]

Actually didn't Andy or someone have a year's worth of data which shows that us about right for LiFePO batteries?

[Moxi]

Cheers Nowty,

Just using your example to show my wife that outside HWC is possible in the UK if not (yet) common. I also got on to the heat geek map team to get them to change the status of my home address from flats to house so that I can start to use their tools to develop my set up a bit more.

Using the current cool weather (3 and -1 today as an example), I have turned down the radiator flow temperature to 38 degrees and the DHW to 40 degrees without telling the wife. This has been in effect for about a week now and yesterday I asked her if the shower was still warm enough and sink water was ok and heating satisfactory.

She gave me a Paddington stare and said yes the heating and water is fine "what are you up to"?

So I explained that I was a) getting the condensing boiler to operate more efficiently by allowing it to condense and b) I was checking the flow temperatures for the house to get an insight into the thermal efficiency of the cottage.

I think I am right in saying that based on this loose check that the radiators are big enough to maintain 20 degrees in cold weather without needing to be upsized and that the flow temperature for the system at 40 degrees is well within the capacity of a heat pump. The next stage is to size the actual heat pump to ensure its big enough but not oversized to deliver the CH and DHW load, for that I need to get the local heat geek engineer in to do his assessments, I did a rough online set of calculations a while back and came in at 7kW, the HG cheat sheet suggests 4.8kW assuming 50W/m₂

[Moxi]

Cheers Nowty,

Just using your example to show my wife that outside HWC is possible in the UK if not (yet) common. I also got on to the heat geek map team to get them to change the status of my home address from flats to house so that I can start to use their tools to develop my set up a bit more.

Using the current cool weather (3 and -1 today as an example), I have turned down the radiator flow temperature to 38 degrees and the DHW to 40 degrees without telling the wife. This has been in effect for about a week now and yesterday I asked her if the shower was still warm enough and sink water was ok and heating satisfactory.

She gave me a Paddington stare and said yes the heating and water is fine "what are you up to"?

So I explained that I was a) getting the condensing boiler to operate more efficiently by allowing it to condense and b) I was checking the flow temperatures for the house to get an insight into the thermal efficiency of the cottage.

I think I am right in saying that based on this loose check that the radiators are big enough to maintain 20 degrees in cold weather without needing to be upsized and that the flow temperature for the system at 40 degrees is well within the capacity of a heat pump. The next stage is to size the actual heat pump to ensure its big enough but not oversized to deliver the CH and DHW load, for that I need to get the local heat geek engineer in to do his assessments, I did a rough online set of calculations a while back and came in at 7kW, the HG cheat sheet suggests 4.8kW assuming 50W/m₂

and 96square meters of floor. Even if I went towards 75W per meter square thats 7.2kW so I am hopeful its in that ball park.

Moxi

[Moxi]

Cheers Nowty,

Just using your example to show my wife that outside HWC is possible in the UK if not (yet) common. I also got on to the heat geek map team to get them to change the status of my home address from flats to house so that I can start to use their tools to develop my set up a bit more.

Using the current cool weather (3 and -1 today as an example), I have turned down the radiator flow temperature to 38 degrees and the DHW to 40 degrees without telling the wife. This has been in effect for about a week now and yesterday I asked her if the shower was still warm enough and sink water was ok and heating satisfactory.

She gave me a Paddington stare and said yes the heating and water is fine "what are you up to"?

So I explained that I was a) getting the condensing boiler to operate more efficiently by allowing it to condense and b) I was checking the flow temperatures for the house to get an insight into the thermal efficiency of the cottage.

I think I am right in saying that based on this loose check that the radiators are big enough to maintain 20 degrees in cold weather without needing to be upsized and that the flow temperature for the system at 40 degrees is well within the capacity of a heat pump. The next stage is to size the actual heat pump to ensure its big enough but not oversized to deliver the CH and DHW load, for that I need to get the local heat geek engineer in to do his assessments, I did a rough online set of calculations a while back and came in at 7kW, the HG cheat sheet suggests 4.8kW assuming 50W/m₂

and 96square meters of floor. Even if I went towards 75W per meter square thats 7.2kW so I am hopeful its in that ball park.

Moxi

I had to edit as I hit post before i was ready

[CharlieB]

Round trip efficiency of a battery is quite a lot better than 75% I would think?

Ok, yep a bit higher. (I was thinking utility scale batteries, which need air con.) but still 10-20% losses. And ‘high grade’ electricity that’s being lost, not ‘low grade’ heat.

In any case, I’m going to carry on assuming that a li-ion battery isn’t the answer for every home. Does anyone know of a rule of thumb for how many Wh storage in a litre of water?