Suburban Heat Networks
Can I make the maths add up to get my local residents to switch to district heating, in suburban South London?
And if it falls short, what government incentive would carry it?
- Seasonal storage located under car-park.
- Run pipework along gardens
- Minimise pipework lengths as far as possible
- Simple HIU to recover cylinder (The SLIM HIU)
- Network connects directly into existing boiler flow and return
- Maximise solar thermal
- Use some of existing gas boilers as backup and peak load backup
- Use heat pumps for bulk of load
1km pipework = 100 properties.
10m / property average.
Managed storage does however appear to work fairly well. 86% efficiency.
Full network losses at 85C are 45kW.
This would be when driving all central heating 5kW average. 100 properties = 1000kW = 9%losses.
So, thermally, the network seems quite feasible. 20% or so losses should be made up by benefits of central plant and seasonal storage.
Looking at 6" pipework, although a different pipe layout with a few interconnections would reduce pipe sizes dramatically. Also there is 17,000 litres in pipes. Maybe less properties would be more efficient.
Only 3,400 litres in network. 76mm pipes, but more like 50mm with it in a loop. Seems more reasonable.
Cost of 43 x heat pump installations = £100,000 ?
Cost of 43 x HIU installations = £ £65,000
165m2 car park. 100m2 with insulation.
6080kWh/year x 43 = 261 MWh.
Note only a 30kW boiler running full time is required to drive entire load. So should only need two households to sign up for RHI on heat contributed. ! Check 5300 kWh/year/property matches some records. Whats the RHI payback ?
This also means, if can't make store as big as would like, should just need to sign up a bit more input capacity or fall back on some existing boilers for top-up.
350kW peak load, so would need to keep 30% of existing boilers to feed into network to get it going without centralised heat generation online.
Season store estimate 50 MWh - approx 3 months peak heating load.
Roughly 1 million litres store dropping from 70C to 30C.
So store would need to be 10m deep - almost a cube.
Best surface area to volume ratio is a ball - maybe that would be best approach if engineering is feasible, to reduce heat loss. 13m diameter would be needed. Plus insulation - so need 50% more land access. Maybe just round corners on cube.
Following study has a good deal on storage. Page 68 covers estimated storage costs. Suggests a million litre store would cost approximately £400,000
Analysis of tank costs by Mark Barrett...
Expanded polystyrene (EPS), has a k-value of around 0.033 W/mK.
R = 30.0 m2K/W
Surface area = 600m2.
Heat loss for 1m thick polystyrene = 600 x 0.033 x (70-20) / 1 = 1kW.
If fully hot for 150 days, this would be 3.6 MWh, or 1.4% of annual loads, so looks great.
Whats the cheapest way to tank up 1 million litres at 70C ?
10m depth is 1 bar, so significant pressure at base. Maybe rounded base is advisable. Back pressure from surrounding soil counteracts pressure, however soil mechanics will need closer analysis if depended upon.
- Cost of removing 1200 cubic metres soil.
Idea for a government: Invest in moulded tank panels. Then given them away on DH schemes. Direct investment in efficiency. Economies of scale could make tanks cheap. Standard 1 million litre design serves 40-50 properties. Standardise the Russian way, then go modular for larger schemes.
600m2 of tank surface. If a 1m2 panel costs £10, that's £6000 - not bad. Other way, allowing £250/property = £17/panel. For the £250 each property gets 23,000 litres storage, so sounds like good value. Installation will be more probably.
Allow for 2% expansion = next of 3m diameter, 3m tall.
Tank sits at atmospheric pressure, using Pandora type expansion system to overcome evaporation losses. This uses an air gap with a diaphragm, and a small semi-open vent with condensate trap. Oxygen ingress into system can be eliminated.
£100 per cubic meter to excavate. 250m3 per day max.
Not sure about a sphere. More complicated, and not best use of space. Cylinder looks better. Can have a hard standing at base poured into pit. Sides are just standardised slightly curved panels. Would it be better to use precast concrete slabs as walls. Can you have a custom made excavator, turns around centre axis. Adjusts depth & diameter for required volume. Ream out the pit.
Then again, why not flat walls. Can use welded scaffolding to add rigidity to take water pressure. Polystyrene insulation blocks should be rigid enough to contain pressure as inner wall is to be lightweight and cheap. Soil mechanics may play a role in deciding best approach, as round walls in polystyrene will take soil pressure (arch technology).
You cant bring in tank in one go. Ideally any sections would be the width of a lorry. Applying a number of thin layers would possible offer better reliability.
Ideally, a simple, cheap, automated rig if possible.
Can all the works be completed underground through a smaller entry hole ?
Polystyrene can take 1 bar ... http://www.foamular.com/assets/0/144/172/174/98cf58e1-c3d2-4b6c-beb5-2063215bea18.pdf
Eccleston and Heart, 0121 683 0300, http://ecclestons.com/ - EPS250. email@example.com/
EPS250 crushes by 1% under 1 bar. There are 5 grades, so costs will be based on using 5 grades, 2 metres depth per grade, to match water pressure.
Size: 10m x 10m x 10m Pit EPS250 1250x1000x2500 80 off EPS200 1250x1000x2500 80 off EPS150 1250x1000x2500 80 off EPS100 1250x1250x2500 80 off Price include delivery to London £67000.00+ vat
More expensive than initially estimated. Will revisit sizes, with a reduced thickness at base (where colder water sits). Only need thin layer on base as will be just above ambient temperatures. 100mm will suffice.
- No space for a cylinder. Cant have just a few on HIUs unless they are next to store, or efficiency dies. This is a good target for funding installation of a cylinder where there is not one. Or you just don't get to connect without one, but each one not signed up knocks efficiency a little.
- Something under car-park. Wont know until investigation gets serious enough to find out who knows.
- How deep is bedrock?
How goes efficiency vary from Summer to Winter ?
to get to 55C...
1.7 COP vs 2.7 COP ... 50% more efficient in Summer.
£0.11 / kWh electricity.
COP of 1.7 = £0.065, COP of 2.7 = £0.047
So 50 MWh heated 50% more efficiently = 50,000 x £0.02 = £1000 per year saving (£23/property). Not much.
So storage isn't justified on basis of energy costs, but more on
- load flattening to lower capex on boilers and heat pumps.
- reducing peak electrical load
- enabling use of heat pumps for all properties by centralising - locating heat pump on each property will be almost impossible.
Issue with heat pumps is we will probably need some high grade heat mid-winter, with store at 70C or above. This also helps on pipe sizing at peak. Using some of the existing gas boilers to raise temperatures from 55C to 75C. If we ever get a -5C snap then can kick in boilers to 80C.
Peak load of 350kW, boiler doing 50% = 150kW boiler to cover coldest weather events. i.e. keep 5 or so of the existing boiler stock.
Efficiency chart for commercial heat pump with flow at 55C:
Saving on maintenance costs
43 gas boilers costs £2150 to certify each year (£50/property).
43 HIU visual inspections at £10 every two years costs £215/year.
Saving is roughly £2000 per year across site, or £45 per property.
Managed Heat storage
We are into the density of housing here that makes storage a requirement if we are to get reasonable network efficiencies and beat the use of gas boilers. Further, we need to manage the storage, using all the intelligence we can to let the network go cold or weather compensate as much as possible.
We are aiming for it to only run 15% of the time in summer to recharge stores. That's around 3 hours a day. Simple to achieve if stores sized correctly and system is managed/automated.
For efficiency, PHE input is required to stores - HIU plus hot water cylinder basically.
£1500 / property for equipment.
UK Power Networks
Projects Gateway 0203 324 1460.