Archive for Downsizer For an ethical approach to consumption
 


       Downsizer Forum Index -> Energy Efficiency and Construction/Major Projects
Brandon

heat store vs cylinder

Right then, before I get the lectures on the different applications, this question arose from a discussion at work, prompted by a comment from a supplier.

Presuming that I have equal volumes of water in each, an they are at the same temperature, and I draw off say 20 litres of water from each, and the cylinder re-fill, and the heat store just sits there, which body of water will be warmer?

If I was to fill a bath, with the same pair, which would provide me with more hot water? i.e. which would give me cold water first, having delivered all its heat?

whilst I'm here, I have a client who wants SDHW but only if the pump is run on PV, so any help on pumps there would be great, as I will need a tidy 12 or 24v pump.

Cheers all
dougal

Re: heat store vs cylinder

Brandon wrote:
... this question arose from a discussion at work, prompted by a comment from a supplier.

Presuming that I have equal volumes of water in each, an they are at the same temperature, and I draw off say 20 litres of water from each, and the cylinder re-fill, and the heat store just sits there, which body of water will be warmer?

If I was to fill a bath, with the same pair, which would provide me with more hot water? i.e. which would give me cold water first, having delivered all its heat?

The key to understanding this is to distinguish between "heat" and "temperature".
To compare with motion:
if temperature is like speed
then heat is like momentum: the quantity of heat is determined by what 'thermal mass' is at that temperature, just as momentum is determined by how much mass is at that speed.

The design difference between either a thermal store or heat bank and an ordinary hot water tank is that the bank/store is designed to minimise mixing (ie "to promote stratification").

In the situation you discuss, the tank will have its heat more evenly mixed throughout the tank, and so will have a more uniform (lowish) temperature.
The bank/store will contain the exact same quantity of heat, but in two distinct chunks, the top very close to the original temperature, the bottom rather cold - but having started with the same amount of heat, withdrawn the same amount of heat, and added the same amount of cold water, so the heat energy content is the same - because the bank/store tries not to mix hot and cold, the next delivery from the bank/store should be at a higher temperature than from the ordinary tank.

The store tries to keep the added cold as a distinct, unmixed, second "body of water" at a different temperature - even though, considered as a single body at a uniform temperature, it would be the same as the simple tank. The design skill is keeping the two chunks from mixing!

Brandon wrote:
whilst I'm here, I have a client who wants SDHW but only if the pump is run on PV, so any help on pumps there would be great, as I will need a tidy 12 or 24v pump.

Have a look at
http://www.alternativeenergystore.co.uk/item-item-103001-1-0-Laing_Ecocirc_D5_solar_DC_Pump.htm
RichardW

What about the tanks that store the heated water but have a coil for the tap hot water so you get mains presure hot water?

Justme
dougal

Justme wrote:
What about the tanks that store the heated water but have a coil for the tap hot water so you get mains presure hot water?

Again, they are designed to use the hottest water in the store to heat the domestic hot water, and keep the cooled (by heating the water heading for the taps) store water out of the way and unmixed as far as possible.
Any lack of mixing (ie any stratification) in a conventional tank is simply fortuitous. It is expected that the tank thermostat switch will call for more heat from the boiler before the average temperature of the conventional tank falls unacceptably low.
Brandon

thanks dougal, that is what I thought, and just needed some other thoughts, as I had a rather confusing bit of advice from a supplier who makes both cylinders and HS.

Thanks
       Downsizer Forum Index -> Energy Efficiency and Construction/Major Projects
Page 1 of 1
Home Home Home Home Home