Helping to heat the house

[graham1]

I have a 1 metre square porch which gets up to about 80degrees in the sun. I am wondering if I can utilize that heat later in the day.

If I use concrete blocks, or similar, to store the heat during the day would there be enough stored heat to transfer into the house to make a difference or would the heat in the blocks be absorbed by the colder evening temperature faster than the heat is transferred into the main house?. Is this a wild cheapskate idea! Any comments would be gladly received

Graham

[Nickfromwales]

6 minutes ago, graham1 said:

I have a 1 metre square porch which gets up to about 80degrees in the sun. I am wondering if I can utilize that heat later in the day.

If I use concrete blocks, or similar, to store the heat during the day would there be enough stored heat to transfer into the house to make a difference or would the heat in the blocks be absorbed by the colder evening temperature faster than the heat is transferred into the main house?. Is this a wild cheapskate idea! Any comments would be gladly received

Graham

Hi Graham.

The only reason it gets up to that temp is because it's a heat trap, and the heat cannot escape. If you tried to tap that heat off for directing elsewhere you'd cool that space in seconds, and would have spent a lot of money on an idea which would produce zero yield. If this were a certified PassivHaus then there would be some useful energy taken from this and added to the math, but not for a regular brick / block dwelling I'm afraid.

[Mike]

If you had a much bigger wall it might be worth it, but not for 1m². See Trombe Wall (Wikipedia).

[SteamyTea]

Assuming 80⁰F

Then the energy in that air is, if it started at 10⁰C is 41 kJ

That is about a third of the energy that the cup of coffee I have just bought, 1,100 kJ.

Even if it is 80⁰C, that is 500 kJ. 

Half the coffee.

Now let us assume that the solar collector area is 2 m² and you are getting 1000 W/m² today.

2000W, or 2 kJ/s.

To heat 41 kJ will take 20 seconds, 500 kJ, 250 s, or 4 minutes.

 

[S2D2]

You can, but only by storing the heat in the house (i.e. overheating it). It wouldn't be worth trying to store it in the porch as there is so little capacity and the losses are huge when the sun goes down.

 

I use a conservatory in this way if heat demand and sun intensity are both high. Open the doors and let the house heat up to 24 then close them. It's up to you and the thermal behaviour of the house as to what you consider a comfortable maximum.

[graham1]

On 29/05/2023 at 22:14, Nickfromwales said:

Hi Graham.

The only reason it gets up to that temp is because it's a heat trap, and the heat cannot escape. If you tried to tap that heat off for directing elsewhere you'd cool that space in seconds, and would have spent a lot of money on an idea which would produce zero yield. If this were a certified PassivHaus then there would be some useful energy taken from this and added to the math, but not for a regular brick / block dwelling I'm afraid.

 

[graham1]

Thanks for your replies. However I ommited to add that as the porch is not very used I was thinking of putting some cement block in there - Have space for about 20 or so  do you think that they would absorb enough heat to have much effect?

Graham

[SteamyTea]

32 minutes ago, graham1 said:

Have space for about 20 or so  do you think that they would absorb enough heat to have much effect?

Work out the mass of the blocks, and the surface area.

Concrete has a specific heat capacity of 0.8 kJ/kg and a thermal conductivity of around 2.4 W/m.K.

Add to that, the very small amount of energy that can be absorbed in the room, not going to be worth it.

 

I had a thermally leaky door to my porch, so always kept the connecting door shut. I fixed the thermal leaks, now I don't need to keep the connecting door shut.

Worth remembering that when the air in the room is hotter than the storage medium, the store is cooling the room as it absorbs the excess energy (which as shown us half a coffee cups worth). Most of the time, the air in the porch will be colder than the concrete storage, but only by a K or 2. So not enough to drive a meaningful temperature change.

High mass houses i.e. brick and block, overheat, or underheat, the same as low mass houses i.e timber ones.

The important thing is the thermal conductivity and the associated U-Value.

There may, on a very rare day, be a time when all the energy inputs and outputs align up and give the impression that 'a news system of heating has been invented'.

It is an illusion based on a small sample, and generally not from research done in the UK.  We have seasons because of our latitude, and a variable maritime climate because of our geographical location. You can't change those two.

 

Edited June 1, 2023 by SteamyTea

[graham1]

Thanks!  I had the feeling that it would not work but I think your science ha supported my gut feeling!