Thermal store sizing for oil plus solar/hydro dump loads.

[Rendall]

Hi all - Our project has been slowly ticking along over the course of this year. The power and heating equations have remained a big areas of questions. Being fully offgrid we need to find the most cost effective/simply designed approach that meets our expected needs - a circa 25-30kWh per day electrical demand, and a heat loss of around 15-20kW plus DHW.

 

We had further quotes from a mains connection in conjunction with some neighbours in our valley, but this is too high a capital outlay (£168k+). We have a stream which present a hydro opportunity so have looked hard at options here in a couple of different ways given that this has such prime potential for electrical generation. The challenge is that whilst the winter flows of the hydro mean we could have up to a 20kW system this would be far more than we need at peak (290kWh per day for around 15% of the year - 55 days), whilst at the other end of the spectrum the driest flows would generate little to nothing for another 15% of the year. The more cost effective solution seems to be a Powerspout turbine which generates up to 1.5kW, but you can also set them up in parallel to increase this.  This would still not operate in the driest 15% of the year, but with two turbines we could have a max generation of up to either 70kWh per day across around 85% of the year.

 

So our thinking is that we have a set up which is around 5kW of solar, a backup generator and batteries. For the small hydro permissions and monitoring is likely to take longer  so we see this a second phase installation we would then need to graft on to the initial system. Essentially it should balance the solar for the wetter/darker periods of the year and cut down on the generator demands.

 

If we had gone with the larger hydro, we had wondered about the electrical capacity opening up heat pump options for heating. However given that even the larger hydro could have outages or dry/cold periods, we'd need something like the Grant hybrid oil/heat pump to have a contingency. However we are then paying for a premium heat pump product, as well as the higher capital investment for the larger hydro.

 

From a cost/simplicity perspective it therefore feels that an oil boiler remains the way to go forward with the heating and hot water. I do have reservations on oil given the environmental direction of travel, but I think it does give us a cost effective solution at the current moment for the likely capital investment with then an absorbable ongoing running cost (and the fact a local supplier does 4x4 deliveries and can deliver to us as we're a couple of miles up a slate track). In 10-15 years when the boiler needs replacing we'll have to see where the wider world of heating technologies has got too as well as what we can afford by then.

 

Thinking is that we'd link this to a thermal store which can also take dump loads from the solar in summer, but then the hydro when that is added on in 'phase 2.' However I'd appreciate thoughts on this and how to best size it. It seems the view is the larger the thermal store, the better. It would need to work effectively across scenarios of a mix of all, or just one or two aspects of the oil boiler/solar dump/hydro dump during either the first phase when we're just running on solar (in which case any dump load would be very limited) or seasonal permutations in the later phase when the hydro is grafted on to the system.

 

Thanks all.

[PeterW]

Any reason you’ve discounted something like a log gasification boiler ..?? 

[SteamyTea]

Leaving aside the size of the individual components for a while, have you considered a combination of PV, CHP (probably home made), with thermal and electrical storage.

The bulk of summer power would be from the PV, with the excess charging up the TS and a relatively small battery bank (4 kWh or so).

Then in the winter the bulk is delivered by the CHP (a 20:40 split between electricity and thermal maybe). Again excess stored. The generator can be rectified to DC, and as long as the voltage us low (<600V I think) it can use the PV islanding inverters).

A CHP system can run off bottled gas, or even gas from a bio digester.

Really depends on how hands on you want to be. Making a CHP unit can be an interesting side project. Making a sound proof 'box' for it will be even more interesting, a hole in the ground may help.

Edited December 27, 2020 by SteamyTea

[A_L]

5 hours ago, Rendall said:

a circa 25-30kWh per day electrical demand, and a heat loss of around 15-20kW plus DHW.

 

1) Can you detail how the electrical demand is made up? For an ordinary off-grid house it is unusually high. IMHO you are going to need at least two days worth of batteries. Also remember that electricity consumed for appliances then turns up as a 'casual' heat gain within the heated volume.

2) Is the heat loss rate really an instantaneous 15-20kW? or do you actually mean 15-20kWh per day, in which case it would be covered by the casual gain referred to earlier.

If the former can you describe the property? Insulation and draught-proofing would seem to be called for.

3) I would meet the DHW by using a small dedicated ASHP along with Waste Water Heat Recovery (WWHR) if your space heating demand is actually 15-20kWh per day.  If your heat demand is 15-20kW then DHW would be heated as a by-product of the space heating solution.

4) Electricity to be provided by Hydro/PV with an air cooled backup generator (Lister)

[Rendall]

16 hours ago, PeterW said:

Any reason you’ve discounted something like a log gasification boiler ..?? 

Mainly the need for daily loading. We do have a lot of accessible timber, but mainly sitka which I once read is not great for log biomass as it creates more residue on the flues (Not sure if that is the reality though in anyone's experience?) Ideally too we'd house the plant for a boiler in a back room of the house, rather than build an ancillary structure for the plant. Hence an oil boiler gave automation and just needed the tank siting outside the back of the house. We did look at automated pellets or chip systems, but the access for local suppliers was an issue as they would only be able to deliver in bulk bags.

[PeterW]

I’ve seen a big chip boiler that is fed from spruce and no issues. It has an auto hopper that takes 5 tonnes at a time and heats a 10,000 litre thermal store. Twice a year they bring in a lorry mounted chipper and that gives them enough chips for the next few months. If you have access and you’re remote from services then this may give you the option for CHP also. 

[Rendall]

13 hours ago, SteamyTea said:

Leaving aside the size of the individual components for a while, have you considered a combination of PV, CHP (probably home made), with thermal and electrical storage.

The bulk of summer power would be from the PV, with the excess charging up the TS and a relatively small battery bank (4 kWh or so).

Then in the winter the bulk is delivered by the CHP (a 20:40 split between electricity and thermal maybe). Again excess stored. The generator can be rectified to DC, and as long as the voltage us low (<600V I think) it can use the PV islanding inverters).

A CHP system can run off bottled gas, or even gas from a bio digester.

Really depends on how hands on you want to be. Making a CHP unit can be an interesting side project. Making a sound proof 'box' for it will be even more interesting, a hole in the ground may help.

Thanks - interesting thought. Not looked at CHP and just been looking at Viessmann Vitovalor (a home made one is far beyond my skills). LPG here would only come in bottles - again due to the access - so I suspect we'd get through a a large number so the oil automation aspect remains a plus in that regard. However the benefit of being able to mix and maximise the heat and electrical generation for the wetter/darker seasons looks like it could be an option. 

[SteamyTea]

55 minutes ago, Rendall said:

LPG here would only come in bottles

 

55 minutes ago, Rendall said:

the oil automation aspect remains a plus in that regard

How come, are the trucks different sizes?

[Rendall]

18 minutes ago, SteamyTea said:

 

How come, are the trucks different sizes?

 Yes, we can get oil in 1000l deliveries by 4x4.

[SteamyTea]

47 minutes ago, Rendall said:

 Yes, we can get oil in 1000l deliveries by 4x4.

Right, then it makes sense to get a diesel powered CHP.

It tis really only an old car engine and a generator put together.

The clever parts of a CHP is the autostarting and phase matching, current sensing autostart would be useful, but your batteries would cover a lot of that.  Phase matching is not necessary as you are off grid.