Willis heater and thermal store

[Huckleberrys]

Hello.  I am toying with the idea of a Willis heater to heat my slab during cheap rate electricity. What are peoples thought on also during that time heating a thermal store of maybe 200litres with a immersion heater so then if a top up of heat is required later in the day it can come from the thermal store rather than the Willis? 

[Huckleberrys]

My solar pv could also heat the store during day which could help at night via a series of thermostats and motorised valves.

[SteamyTea]

11 minutes ago, Huckleberrys said:

200litres

That will give you about 9 kWh of storage (4.2 [kJ/kg.K] x 200 [kg] x (75 - 35) [dT] * 0.00027778 [kJ to kWh] = 9.3.

So probably useful, would do my 50 m² house.

[JohnMo]

Set the correct flow temperature and number of hours, as long as our screed is thick enough to store the energy required. Works well with a thick screed not so well a thin screed.  Then it's a matter of matching heating time to likely demand due to outside temperature.

 

If you're putting in one Willis, install a spare at the same time, as they are cheap.

[Huckleberrys]

In theory it will be in a 100mm screed with a house heat loss of 1.3kWh. 110m2 single storey. I just thought the thermal store would be a useful addition to save using expensive electricity if a top up is needed during the day. I'd have to run the sums to see if the extra outlay is worth it against the cost of the electricity.

[SteamyTea]

20 minutes ago, Huckleberrys said:

house heat loss of 1.3kWh.

Probably isn't.

Taking 1.3 kW as the most likely heat load, then 9 kWh will supply 7 hours of heating.

Now assuming that your cheap rate is also 7 hours, that is 14 hours of heating at the cheap rate.

36 minutes ago, JohnMo said:

as long as our screed is thick enough to store the energy required.

If you have intermittent energy input i.e. cheap night, then to get the mean temperature you desire, there will be times when the slab is over temperature.

With energy stored in water, you have more control i.e. pump when needed.

 

There are downside to both systems, so I would look at the overall system efficient which is really down to how much the slab looses to the ground for each increase in dT.