Are we targeting ASHP's at the wrong market?

[Beelbeebub]

That CO2 per kwh thermal of natural gas is about 200g/kwh.  

 

This tallies with around 220g per kwh(thermal) for a 90% efficency boiler. 

 

CCGT plants are upwards of 50% efficient at converting thermal energy to electricity. Which tallies with the 400g of CO2 per kwh (electrical). 

 

With flow temp of 45c or less most modern HPs can achive better than 2:1 down to well below 0C.

 

Even at 55C flows they achive 2:1 down to -2C

 

There are valid reasons why we can't replace every gas boiler with a HP but "you will increace co2 emissions from electricity by more than you will save by stopping direct burning" isn't one of them. 

 

[SteamyTea]

Can we try and stick to convention for the units.

 

kWh for energy

kW for power

_e for electrical

_t for thermal

 

When discussing heat pumps, we really should use the kelvin [K] temperature scale.  This becomes important if you have to multiply or divide, it gets rid of that messy problem when expressing efficiency as a percentage.

[LnP]

4 hours ago, marshian said:

When I worked for British Sugar we had boilers producing superheated steam to run a steam turbine to generate electricity - the “waste” steam was used to boil the sugar solution under vacuum until it crystalised and was spun off from the molasses 

 

It’s been a while since I was in that game but I guess the advantage was both the electric generated and the steam post turbine were fully used - even the condensate was recovered and put to use.

Current work I was pretty sure our CHP unit (Combined Heat & Power) does better than that too (gas powered monster of an engine that runs a generator to produce electricity but again we use the engine exhaust heat to support our steam generators that also run on gas but have massive heat recovery systems on the flue gases. 
 

Still with a monthly gas bill the wrong side of £250,000 we need to minimise any waste in the system.

 

When gas prices spiked as a result of the Ukraine situation our gas bill hit over half a million per month - that really screwed with our costs to manufacture products!!!!!

 

I’m honestly surprised that a gas powered electricity generator that supplies the grid is as poor as 400g of CO2 per kWh generated - normally generation at scale is a lot more efficient

 

 

The laws of physics (2nd law of thermodynamics) determine that for every 100 kWh of gas you burnt in your boilers, the best you could ever have got out as electricity would have been about 64 kWh. The other 36 kWh was coming out as waste heat (first law of thermodynamics, energy can neither be created nor destroyed). So, it's good that your plant found a way to use that waste heat. It's the same story for a CCGT power station but unfortunately it's not so easy to make good use of the waste heat. District heating would be one way if there are users nearby.

 

400 g CO₂ per kWh is about right and will never approach the performance of a gas boiler.

 

I'm not an expert on CCGT, so I'm not sure about what benefits you can get by generation at scale. But regardless the scale, the maximum you can get out as electricity is the Carnot efficiency, which is 1-Tc /T_h.. T_c is the temperature on the cold side T_h is the temperature on the hot side. If say T_c is 300 K (27 ^oC) and T_h is 600 K (327 ^oC) the maximum efficiency you can get, limited by the laws of physics, is 50%. With a gas boiler you can get close to 100%.

 

Nevertheless as @Beelbeebub pointed out, if you put the electricity into a heat pump, for a given amount of energy going into heating your home, overall you'll get about half the CO₂ emissions compared to a gas boiler.

 

 

 

 

[LnP]

5 hours ago, PhilT said:

Gas plant efficiency is far worse than theoretical because of renewables fluctuations

Not sure about this ... as far as I know, the reason CCGTs are seen as a good fit with renewables is their flexibility to be turned up and down as the generation from wind and solar varies. But as I mentioned elsewhere their efficiency (kWh of gas in to kWh of electricity out) can never beat the laws of physics.

[Beelbeebub]

The effect on CCGT efficency of any renewable fluctuations is fairly irrelevant to the "but more co2" argument.

 

In order to produce less CO2 per kwh(t) than a gas boiler, HPs need only hit 2:1 efficency which is almost always the case for UK temperatures. 

 

Even for the few days a yer that the HP's might drop below 2:1 and the instantaneous co2 emissions go above those of a gas boiler, the annual efficiency (aka SCOP) of a HP will always be well above 2 (for any reasonably installed system).

 

And the above calculation assumes *all* electricity was produced from gas ie the grid carbon intensity is 400g. In reality the grid intensity is far below that. For example the worst week of 2024 for grid co2 emissions was 220g.  So for all of 2024 a resistively heated house produced less carbon than an equivilent gas boiler heated house.