Heating Homes With Tap Water
Can tap water be used to heat homes? It’s not as crazy as it first seems - the energy released when water is frozen can be extracted by a heat pump and used to keep homes warm. This could be a very efficient source of heat for apartments and houses in the city.
The UK faces a huge challenge. 22 million houses are heated with gas - accounting for 77% of heating CO2 emissions. To reduce emissions, we need to change this.
The simplest solution is direct electrical heat. However, this is highly inefficient - although an electrical heater is 100% efficient - that is, all the heat it produces is used to heat your house - generating and delivering that electricity is only 35-55% efficient. If we have to burn fossil fuels to produce that electricity, more will be used than if we’d just stuck with gas heating.
The best solution for decarbonised electric heat is the heat pump. It works like a fridge in reverse - moving heat from a “heat source” into your house. The heat source is cooled, the heat is concentrated (raising the temperature) and then distributed through hot water.
By moving heat from outside, heat pumps can put more heat into your house than the electricity they use. This ratio is called the “coefficient of performance”, and it’s typically 3-4x. This makes them ideal as a way of heating homes while using minimal energy.
Depsite being a “heat source”, the heat source doesn’t need to be hotter than the house - in fact it’s typically anywhere from -10 to 20 °C. What it does need to be is regenerating - the heat pump is sucking heat out of the heat source, and so it must be getting warmed back up by its environment.
Typically the source is either the ground (“ground source”) or the air (“air source”) - although other sources can also be used. Some houses use a heat source loop sunk in a pond or lake, others use a stream. Any source of heat can be used, as long as it can maintain its temperature as energy is extracted.
This requirement for a heat source is a problem. Ground source heat pumps are only suitable for a small fraction of houses - those with large gardens or ponds. Most homes in the UK don’t have this. That leaves air source heat pumps, which work a lot like an air conditioner in reverse.
This isn’t ideal either.
- They need large, noisy, unsightly fans on the outside of buildings
- The efficiency drops as the air temperature drops - meaning when heat is needed most air source heat pumps are not efficient and struggle to provide enough heat.
Using the heat in tap water
An interesting source of heat, which is available in every boiler cupboard already, is tap water. If the water is cooled and partially fused into ice, heat from both cooling the water (the specific heat) and fusing it into ice (the latent heat of fusion) can be extracted by a heat pump.
This is essentially a form of ground source heat, but in reverse. Rather than pumping heat from the ground, heat is removed from the water first, and then returned to the water when it enters the underground sewage network.
Using tap water as a source of heat is interesting as it requires none of the invasive building works common for heat pump installations. A mains water connection and a drain are already present alongside every condensing gas boiler. A little bit more space will be required for the heat pump, buffer tank and heat extraction machine, but no other works would be required to install such a system.
Doing the maths on an average apartment
Having done the sums, I think this concept could work. The amount of water needed isn’t completely crazy. It’s not a free lunch - you trade energy consumption for water consumption. And there’s bound to be problems with making all that ice. But these issues seem manageable.
I couldn’t find any discussion on source of heat anywhere on the web. To answer this for myself, I needed to calculate a few things.
- Water use when heating
- Total water use each year
- How it compares to existing use
- The energy needed to distribute the water
Only then can we answer the question does this make sense?.
Case study details
We’re going to look at the Vaillant geoTHERM Mini 3kW - a small heat pump designed for apartments and small houses. It uses a brine loop to source heat - normally from a ground source.
|Water 35°C, Brine 0°C (ΔT 5°C)|
|Heating output||2.5 kW|
|Power consumption||0.7 kW|
|Coefficient of Performance||4|
| Water 55°C, Brine 0°C (ΔT 8°C) | | |--------------------------------|--------| | Heating output | 2.2 kW | | Power consumption | 0.9 kW | | Coefficient of Performance | 2.5 |
The CoP varies depending on the output water temperature - from 4 at 35°C to 2.5 at 55°C. The heat pump draws between 0.7 and 0.9 kW from the electrical grid, and up to 1.7kW from the heat source.
Water use when heating
The specific heat capacity of water is 4.2 kJ/litre - that means by every degree a litre is cooled down you can extract 4.2 kJ. The latent heat of fusion of water is 334kJ/litre - that means every litre of water you freeze, you can extract 334 kJ.
Tap water averages 7.3C - dropping as low as 1.8C during extremes of weather. Fortunately the temperature isn’t particularly important - the latent heat of fusion gives much more energy than the specific heat of the water anyway - the difference between 10 degree tap water and 1 degree tap water is only about 10%.
To source 1.7kW from the heat of fusion of water, that means we need to turn (1.7 / 334) = 0.0051 litres/second of water into ice. Because we can’t drain away solid ice, the ice needs to be diluted. Commercial slurry ice is typically 25% ice and 75% water - so let’s assume the same ratio.
This means the heat source would have a peak water consumption of 0.0204 L/s, or 1.224 L/minute - about 20% of a running tap.
Total water use each year
The average power usage of a house is a lot less than the peak usage. The average house uses between 5000 and 30,000 kWh of energy each year for heating. Our case study is for a small apartment, so we’ll use the low end.
1 kWh is a measure of energy - as is kJ. In fact 1 kWh equals 3600 kJ. So our house uses (5000 * 3600) = 18,000,000 kJ/year, or 18 GJ/year.
The heat pump coefficient of performance tells us that for every unit of energy which comes from electricity, we produce between 2.5 and 4.1 units of heat. By reversing the calculation, we can work out that for every unit of heat into the house, we need to extract between 0.6 and 0.76 units of heat from the water.
That means we need to extract between 10.8 and 13.7 GJ/year from tap water. We can then work out the total ice we need to produce - between 32,300 L and 41,000 L, or 88.5 - 112.3 L/day. Let’s take the average - 36,500 L/year or 100 L/day.
As we need slurry ice (so it doesn’t clog the drains), this value needs to be multiplied by 4. That gives 146,000 L/year, or 400 L/day.
How it compares to existing use
An average household uses 349 L of water each day - or 127,385 L/year. So over a year we’d use an additional 115 % of water.
That’s a fair bit more, but it’s a feasible amount to deliver with the existing water infrastructure.
If the household greywater could be used to dilute the ice, rather than fresh clean water, the total water consumption could be shrunk significantly. In the best case, where all grey water was used to dilute the slush, the increase in water consumption would only be 29%.
The energy needed to distribute the water
Establishing the energy and environmental cost of fresh water is complex. It’s very dependent on location - water is far more valuable in a desert than a rainforest and far easier to distribute in an area with high mountain reservoirs than a flat one. However, we can find a range of numbers.
The 2015 American report “A survey of Energy Use in Water Companies” suggests treating and distributing water takes 1,500 - 3,500 kWh per million US gallons, with a mean of 2,300 kWh/million gallons. That is 608kWh/million litres, or 0.6 Wh per litre.
In the UK, Thames Water’s deliver 2.6 billion litres a day, while 22% of their total energy usage is 298 GWh (that’s the renewable portion apparently). That would imply 1354.5 GWh/year - which gives an energy usage of 1.43 Wh/litre.
Taking the larger number, producing the additional 146,000 L/year of water for this proposal would use 209 kWh/year. That’s about a 4% increase in power usage compared with a ground source heat pump. Considering an electric heater would use 400% more power this isn’t a huge amount.
Even desalinated water would make sense - reverse osmosis needs 6.8 - 8.2 kWh/kgal, which is about 1.8 - 2.2 Wh/litre - only another 5-6% energy usage.
Does this make sense?
I think so. The amount of water used is around twice the current consumption. Would the infrastructure cope? Almost certainly - as water pipes (like any distribution network) have to be sized for peak demand, excess usage could easily be taken at times of low demand. This system wouldn’t make sense for every house, only ones where a ground source or air source heat pump couldn’t fit.
The biggest problem I can see would be managing the ice production. It would have to be pushed out the house before it melted, otherwise it would just suck back the energy that was taken from it. If the drains froze up (or the ice didn’t melt - particularly a problem during cold snaps) then the blockage could stop the heat pump. This might be a hard problem to reliably solve. Flushing it out with the other household greywater would help immensely - but this would often require reconfiguring drain pipes.
But if we could solve it, the benefits are huge. A system using tap water as a heat source could be installed in place of an existing condensing boiler with no extra connections or wider works. It would need more space - for the ice making system and a buffer tank - but no groundworks or lifting of floors. Basically none of the expensive things that are stopping us right now. Given that we have to retrofit 22 million homes in the UK alone, this would be a huge advantage.
Have I missed something? Made a mistake? I’d love to know if you disagree, or agree, and why.