Green Building Forum - Xsorb inter-seasonal heat storage!

Xsorb inter-seasonal heat storage!

Chris P Bacon — Fri, 04 May 2012 14:03:40 +0100

From what I have read on the subject David, solar air panels typically get a 30degC uplift in air temperature on a good sunny winters day but the efficiency falls off drastically if you send the hot air around for another pass. Ambient temperature and consequent heat loss from the panel being the practical limiting factors.

Assuming you are recirculating air from within the house at 18-20C and it's coming off the panel at 50C we currently don't know by how much this will charge the store.

I've just had a quote from a Solar air panel supplier and from that I would estimate that at my latitude (55.3N) you will need between 40-50m2 of Solar air panels to satisfy the gross December heat demand of a 150m2 certified passive house. (depending on how well optimised the panels are sited)

Then it is a question of the efficiency of the thermal store in charging and releasing this heat.

If the efficiency is reasonably high then it would appear plausible to have a reasonably small (1 -2m3) Xsorb thermal store which if fully charged going into the heating season could keep you warm for the winter.

It all depends on the efficiency of winter recharging and assumes you don't need huge fan power across the store.

Xsorb inter-seasonal heat storage!

djh — Fri, 04 May 2012 14:39:29 +0100

I just tried some sums, for an example passivhaus. Feel free to change any of the numbers if you don't like them.

floor area = 100 mÂ²
ceiling height = 2.5 m
so volume = 2.5 x 100 = 250 mÂ³
internal temp = 20 C
specific heat load = 10 W/mÂ²
So heat load = 100 x 10 = 1 kW

0.25 < ventilation rate (ACH) < 0.5
(note that the specific heat load goes up noticeably in PHPP as you increase the ventilation rate, so you can't just turn up the ventilation arbitrarily)

From volume and ventilation rate we calculate volume of air per second 250 v / 3600 = 0.0174 < mÂ³/s < 0.35

30% < Internal RH < 60%
(and it will probably tend towards the lower end of the scale in winter in a house with mechanical ventilation)

saturation water vapour density at 20 C = 17.3 g/mÂ³

So from RH, we calculate mass of water per cubic metre in range 5.2 - 10 g/mÂ³

And combining the density of water with the ventilated volume rate we calculate the mass of water vapour available per second
@ 30% & 0.25 ACH = 0.09 g/s
@ 60% & 0.5 ACH = 0.36 g/s

Latent heat of vaporization of water, LV = 2260 kJ/kg
(I assume that all the vapour in the air is condensed and the LV recovered. Supposedly more than LV is recovered by the unit but then 100% of vapour won't be condensed)

So available heating power is
@ 0.09 g/s => 203 W
@ 0.36 g/s => 814 W

Note that in practice, the RH will decrease as the ventilation rate is increased, so the 0.36 g/s is pretty unlikely. The water vapour in the air comes from outside (limited by saturation at outside temperature, supply reduces as temperature does) and internally generated by breathing and cooking etc (essentially fixed rate).

On that basis, there probably isn't enough moisture in the air to make the system work. It depends on just how much energy is recovered during the adsorption.

Note that you can't use a humidifier to fix any shortfall, because that will take just as much energy to run as will be recovered by the unit. And you probably need a backup heater for really cold weather.

Xsorb inter-seasonal heat storage!

Ed Davies — Fri, 04 May 2012 15:52:15 +0100

My calculations are very much in line with what djh says.

It's worth noting, though, that the latent heat of vaporization that djh quotes is for 100Â°C - you get an extra 10% or so at around room temperature (see my post half way down the first page of this thread).

We're getting a bit repetitive here but, in addition, the design/assumed heat load of 10 W/mÂ² is calculated on the assumption of a certain level of insulation and airtightness combined with MHRV which will already be recovering some of the latent heat of vaporization in the outgoing air (via condensation). Therefore, if we're to count that heat towards the heating available we must also include it in the heating required (i.e., more than 10 W/mÂ²) so the shortfall is a bit greater than djh's numbers indicate.