When considering water source heat pumps you have to consider that catchment area of that river and not just the volume of water in it.

Clouds and rainfall are just stored solar solar energy.

I have a huge store of solar energy not 2 miles from me, it keeps the temperature stable, but it is, to all intents unavailable because the costs of transporting thermal energy is ridiculously high.

How about 40% of London from the Thames?

10 million Londoners. (https://en.wikipedia.org/wiki/London something between Urban and Urban Zone populations - can't be bothered to look what they mean). Give 'em 2 kW each. Take 40% from the Thames gives a requirement:

>>> r = 10e6 * 2e3 * 40/100
>>> r
8000000000.0 [W]

(i.e., 8 GW)

Flow rate is just under 66 m³/s (https://en.wikipedia.org/wiki/Thames) with 1000 kg/m³ and specific heat capacity 4200 J/(kg·K) so heat capacity rate:

>>> c = 66 * 1000 * 4200
>>> c
277200000 [W/K]

Temperature drop is requirement divided by capacity:

>>> r/c
28.86002886002886 [K]

so, yeah, if you just extracted heat from the Thames at one point it would freeze very solid.

If you're taking heat out distributed over, say, a 10 km length of the River with the width roughly what it is at Chelsea Bridge (https://en.wikipedia.org/wiki/Chelsea_Bridge), say 200 m, then the surface area is

>>> a = 10e3 * 200
>>> a
2000000.0 [m²]

and the power needing to flow into the water to replace that being extracted would be:

>>> r/a
4000.0 [W/m²]

That's still an awful lot.

Posted By: Ed DaviesIf you're taking heat out distributed over, say, a 10 km length of the River with the width roughly what it is at Chelsea Bridge (https://en.wikipedia.org/wiki/Chelsea_Bridge" >https://en.wikipedia.org/wiki/Chelsea_Bridge), say 200 m, then the surface area is

Posted By: Ed Davies4000.0 [W/m²]

That's still an awful lot.

But that's a flawed way of calculating how to get the heat in. The watershed area of the Thames is not just its physical surface, it's much, much larger than that.

What you need to do is calculate the area of watershed required to supply the heat needed (and do it in MJ or kWh, not W) given the average insolation data for the south of England. I think you'll find it's entirely feasible.

Paul in Montreal.

Posted By: Ed DaviesFlow rate is just under 66 m³/s

On reflection, I'm mildly suspicious of that number. If the river's 200 m wide and a couple of metres deep and flowing at, say, 1 m/s then the number would be a lot higher. Maybe 1 m/s is reasonable for the surface away from the banks but is a bit high for the average including the bits by the banks and in the weeds, etc. Dunno.

Posted By: Paul in MontrealBut that's a flawed way of calculating how to get the heat in. The watershed area of the Thames is not just its physical surface, it's much, much larger than that.

Sure, but the water gets heated to whatever temperature across the wateshed then transported to the actual Thames by soaking through the ground and running down tributaries. If you then start trying to extract 8 GW from it you'll drop its temperature by about 30 °C which is obviously way too much. So then the question is how much more heat can you pick up locally to replace that?

Obviously in practice it's not all the same water - water evaporates and more comes in from rivers like the Fleet and Lee and just soaking through the ground. If it was only a few watts or tens of watts per sq metre I'd say maybe but 4 kW/m² seems a bit unlikely even taking that into account.

Posted By: SteamyTeaThe Thames itself provides two-thirds of London's drinking water…

OK, but so what?

Suppose people take 150 litres per day each. 10 million peeps. 1000 litres in a cubic metre. 86400 seconds in a day.

>>> 150 * 10e6 / 1000 / 86400 * (2/3)
11.574074074074073

Less than 12 m³/s. Quite a lot less than the 66 m³/s I was supposing we were cooling.

The specific heat capacity of water is high (compared to almost anything else) but it's not magic. And the Thames is, on a global scale, a pretty tiddly river. And London is, on a global scale, not that populous but it is pretty dense.

yes it does and in times of low rainfall the tide swashes in and out largely with the same water as not much is coming down the river.

Posted By: SteamyTeaWould each individual need 48 kWh[t]/day in a city?

At the coldest time of the year, and including space heating for offices, shops, etc, it doesn't seem ridiculous to me. Got any figures to suggest otherwise?

I think I am getting too used to my mild climate down here. Had the main heating off for a few days again.
I did see some figures for London once, was lower than the national average but can't remember by how much.

Paul
That is about what I use in a month.

Maybe something in this report:
http://legacy.london.gov.uk/gla/publications/environment/soereport/full-report.pdf

Posted By: DantenzWorst case for me was 55kWh electricity consumption over 24hrs for my air/water heat pump when we had -14'C

Right - but that's the electrical input. We're talking about the thermal input to the heat pump. If the COP is greater than 2 then the thermal input will be greater.

It's a fair enough point, though, that the electrical input to the heat pump also contributes.

Posted By: Ed DaviesRight - but that's the electrical input. We're talking about the thermal input to the heat pump. If the COP is greater than 2 then the thermal input will be greater.

No, that was my heat input into the house. Electrical input was around 110kWh for the coldest days. So I guess about 240kWh came out of the ground.

Posted By: DantenzWorst case for me was 55kWh electricity consumption over 24hrs for my air/water heat pump when we had -14'C

The worst case figures for me were daytime highs of around -23C and nightime lows around -28C

Paul in Montreal

There was a website (Canadian) that showed the flow and return temperatures of a WSHP, can't remember where it was, Lake Ontario I think, some University.

According to the document I linked to earlier, the per capita energy use in London was about 22 GWh in 2003. That is for all energy. So if housing uses a third (an assumption) then that will be 0.87 kW per person. Some of that will be electrical, say 30% again (though that is a total guess, it could be 60%), so thermal would be lower at around 0.5 kW or 12 kWh/day.
Though the majority of thermal is needed in the winter.

This document may have more up to date data:
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/244790/september_2013_sub_national_total_final_energy_consumption_statistics_factsheet.pdf?time=0

Struggling with a wriggle python at the moment so don't have much time to look at it.

Yes, does seem high to me though.
If ASHPs where fitted would that cause snowfall, whatever snow is

What would be a typical air temperature drop across the collector at say an indoor temperature of 20°C and an outdoor temperature of 10°C

No it don't

Was just wondering if there had been some research done on it.

Posted By: Ed Davies "Flow rate is just under 66 m³/s...........;On reflection, I'm mildly suspicious of that number. If the river's 200 m wide and a couple of metres deep and flowing at, say, 1 m/s then the number would be a lot higher. Maybe 1 m/s is reasonable for the surface away from the banks but is a bit high for the average including the bits by the banks and in the weeds, etc. Dunno".


I used to go down the Thames in a small boat and found that, leaving Richmond an hour or so after the top of a near-spring tide, the best current would be about 2 knots as far as Kew, then increasing to about 3 kts as far as Greenwich, probably a bit more in the Pool of London. 1 kt is about 0.5 m/s. So, taking account of neap tides, slack water and the reversal of the tide's flow part of the time, the average speed over an average tidal cycle must be well below 1 m/s.

If one could cool the water sufficient to form small ice floes you'd have the benefit of the phase change heat, but it would knock the COP rather. And not popular at Southend, possibly.