Sea Source Heat Pump

[rsmith7]

After burning 400 litres of heating oil in the last 12 days of bitter weather - due to global warming - I'm considering fitting a heat pump to the outflow from my lobster ponds.
We pump 25 - 30 m3/hr of sea water that varies from 3C - 17C.
The pumps run half of the time (3hrs either side of high water).

I doubt normal ground source heat pumps are cost effective but given the above they might be quite good.

Any experts in the field?

[principled]

Not an expert. But the figures (rounded for simplicity) are as follows:
Assuming that your heating oil is kerosene, then it has a heat value of around 28000kJ/litre. As you are using 33 litres a day (and assuming the boiler is 90% efficient), your heat input into the building is around 800MJ.
The heat energy potential (assuming 25m3/hr) in sea water is 100MJ/degree C. So running for 6hrs would give you 600MJ potential for every degree C you drop the water temp. Unfortunately, I have not been able to find any "small" heat exchangers and so I can't tell you their efficiency.
The page below contains a few links about the use of sea water on big heat pump projects. One is a pdf link to a press release by a company(?) called Friotherm about the heating of Stockholm. It occurred to me that a quick note to them could result in names of heat pump/exchanger suppliers .
One thing they must have been overcome is the corrosive effect of the sea water on any metal heat exchanger, which I would see as a major problem.
www.refrigeration-engineer.com/forums/showthread.php?t=6594
P

[marchesarosa]

What a helpful man you are, principled!

Karma!

[rsmith7]

Thanks principled, I'm doing some homework and will report back.

[carnyx]

Try Googling Coefficient of Performance which leads to all sorts of useful stuff.

Also, corrosion is an issue; but these beasts below are designed for the job, with CPs of around 5
(i.e. 1kw of electricity gets you 5kW of sensible heat)

www.drydenaqua.com/acatalog/Titanium_heat_pumps_for_heating___cooling.html


But if you don't have mains electricity and so cannot use offpeak rates, then either a diesel genny or a direct diesel heatpump might be the answer. If you go for a water-cooled diesel genny then the integral radiator could be bypassed by connecting up your central heating system to act as the radiator instead. This would use all of the waste heat giving a theoretical economical efficiency ( electricity plus heat ) of 80%.

Then if you used the electricity to drive your heatpump then you might get an economic multiplier of around 7 KW of usefuk heat per 1kW of heat, rather than just 1 kw if you burned it in a furnace.

Altogether, you have a fascinating project and I suspect grants may be available.

If you have mains electricity, a simple trial could be done using a second-hand freezer, a Central Heating pump, and fresh water circulating through long lengths of plastic hosepipe submerged in the sea (or even a local pond) to act as the heat exchanger. The heat will be given off by the freezer cooling fins, and you ought to see a COP of around 3 ( i.e. 3kw of convected heat per 1 kw of electricity)

[jonjel]

R Smith.

The Festival Hall in London is (or was) heated by heat pumps in the Thames.

It should work, though be prepared for lower efficiency than you might have anticipated.

[jonjel]

Mr Smith.

Is the sea temperature still at +3 today or are you breaking the ice on the stock ponds?

-8 here last night, but no doubt that is positively balmy compared to some of you...

[rsmith7]

Looks like the plant will cost £10 - 15k and use a fair bit of electricity.
I doubt whether the machine will last 10 years - salt water, heat and electricity don't mix very well.
I can see a fair bit of (expensive) maintainance.
Haven't calculated everything accurately but it looks like a waste of time. Break even at best.
Bugger

[jonjel]

I am surprised at that. There are many materials which will not corrode in sea water from which the coils could be manufactured. And not too expensive either.