Design of offgrid provision for Elec/DHW/CH

[Rendall]

Hi All -

 

We've been slowly ticking away with our offgrid renovation project about which I posted a few questions a year or so ago.

 

Biggest design challenge (to date) has been trying to find contractors who can look across the different systems at play and getting to a point where I have confidence that we have a workable, cost effective, and not over complicated, whole. At present the solar guys don't do hydro. The hydro guy does do solar, but not heating. The heating guy does not do hydros. So lots of potential gaps for things to fall through.

 

So I thought I'd share the shape of the main information and questions we have on here and see if anyone has thoughts on the interplay of the systems, or thoughts on the questions we should be digging into or asking further. As I mentioned when posting before, this is a fully offgrid property - no mains gas or electric anywhere nearby so we've essentially got to set up a fully functioning micro-grid.

 

Electrical Generation

Hydro - We can get a pico hydro on the nearby stream which with two powerspout pelton turbines running off the one penstock should give us a reliable 3kw/72kwh per day. We may be able to add a third to this, which may give us an option for extra generation of 1.5kw in autumn/winter.

Solar - In conjunction to this we have a reasonable south facing site which could host 4-6kw of solar.

Generator - We'd have a backup generator linked to the system

These would feed into a lead acid battery system and from there then feed into the house.

 

Electrical Demand

From some rough demand calculations, we think we'll have a daily demand of approx. 40kwh, with a peak load of around about 5kw. (NB I've not yet modelled this based on summer/shoulder seasons/winter to see how different it might or might not be)

 

Cooking

Included in the above figures are an electric Everhot 110 range. (Backup gas LPG bottle fed oven in back kitchen)

 

Hot Water & Heating

The house is a good 360m2 (two main floors of 155m2 plus couple of attic rooms). Construction is slate slab walls approx. 30cm thick. Roof and attic insulation will be put in and north facing rooms will be internally insulated. The house is south facing, but it is a wet climate around here (its the rain belt of mid-Wales). We'd put UFH under the slate slabs in the groundfloor hallways, back rooms and kitchen. Log stoves would be in three other groundfloor rooms. Heating calculations from one heating engineer suggested a need for about a 45kw boiler. A biomass boiler is not practical as the pellet delivery lorry would find the access difficult and we don't want the physical dependency of needing to feed a log supply regularly. So oil is what we have been thinking about for the fuel (and the local firm is used to access up tracks like ours with the range of farm customers they have).

 

Conversations have been that we'd have a thermal store into which we can dump any extra electricity but I'm assuming the amount we can count on to dump might also be able to reduce the potential oil boiler sizing? However, with potential generation of up to 108kwh (if we were to set up three turbines running) we'd presumably need a fairly large thermal store to handle the potential amount of electricity we need to dump?

 

Given the difference between a potentially large regular electrical supply of 72kwh (or indeed more) for the wetter 6 months of the year, compared to a more volatile say c.50-80kwh or so in the summer, might this make sizing the thermal store difficult as it may be far too oversized for what would work effectively in the summer?

 

Separate to all of this might there be any logic in having a backboiler on one of the woodstoves which can also be an extra occasional winter feed to the thermal store?

 

There would be an argument to get the hydro to generate as much as it can and aim to run the hot water and heating fully from this and the solar. Not sure if the numbers would stack up for this or not, but ultimately even if they did, my worry would be what would happen when a part on the hydro breaks and it take a couple of weeks to get a replacement. Having an oil boiler that's also linked into the hot water/heating, plus the back up gas oven means, makes me think that we'll at least be warm and fed, even if we're using candles and undertaking a digital detox.

 

Hope that provides some context on the aspects at play.

[ProDave]

My immediate thought is wow, why is your heating demand that high?  A constant 3kW from hydro would power my entire house with energy to spare.

[Rendall]

@ProDave It's one of the questions I have. I've got someone else working up some calcs on the heating demand and heat loss for me to sense check the 45kw figure that I'd previously been given. I have a sneaking suspicion that that first chap had not done the full calcs but plucked the number out of the air when I chased him. My own BTu calcs came up at 88,000BTU, dropping to 80,000 if I can get some decent double glazing through the planning process. However that was just a basic online calculator and my first introduction to the world of BTUs so I'm not trusting them!

[SteamyTea]

88 kBTU is 26 kWh. (it is kWh not kwh or KWH, or KwH, can see what BTUs are used sometimes)

360 m² is quite large (about 7 times larger than my place and that uses around 10 kWh/day this time of year)

 

If you can really get a reliable 3 kW of power from the pico hydro, then only a third of that is needed for space heating, so bump the thought of combustion technology.

 

Edited February 4, 2020 by SteamyTea

[Ed Davies]

15 hours ago, Rendall said:

… my worry would be what would happen when a part on the hydro breaks and it take a couple of weeks to get a replacement. Having an oil boiler that's also linked into the hot water/heating, …

 

Which is cheaper, a spare hydro turbine and misc repair materials for the penstock, etc, or an oil boiler and tank? A spare turbine on a shelf in a dry room needs less servicing than an oil boiler that's not being used much.

 

4 hours ago, SteamyTea said:

88 kBTU is 26 kWh

 

It can also be 26 kW because, sadly, lots of people use BTU to mean BTU/h.

[SteamyTea]

7 minutes ago, Ed Davies said:

can also be 26 kW because, sadly, lots of people use BTU to mean BTU/h.

Yes, or even Btus, just really confused.

Is a BTU the energy needed to heat a lb of water 1°F?

SI us so much better, but then it is more fun bamboozling people. 

Edited February 4, 2020 by SteamyTea

[jamieled]

One thing that you should consider re the hydro, which I have come across on a couple of pico installations.

 

Most energy projections for hydro use a flow percentile, typically the 95th exceedance percentile, to give an estimate of a reasonably low flow and therefore energy production. It is often assumed this occurs in summer, but depending on where you are in the country it can also happen in winter, typically during high pressure with very cold temps. This starts to freeze up water in the catchment and reduce flows.

 

To cut to the chase, make sure your system can cope with a scenario where it is cold and the hydro cannot work at full whack.

[Rendall]

8 hours ago, jamieled said:

One thing that you should consider re the hydro, which I have come across on a couple of pico installations.

 

Most energy projections for hydro use a flow percentile, typically the 95th exceedance percentile, to give an estimate of a reasonably low flow and therefore energy production. It is often assumed this occurs in summer, but depending on where you are in the country it can also happen in winter, typically during high pressure with very cold temps. This starts to freeze up water in the catchment and reduce flows.

 

To cut to the chase, make sure your system can cope with a scenario where it is cold and the hydro cannot work at full whack.

Yes @jamieled, that's exactly what I want to bottom out. We're about to get the low flows report carried out so hopefully that will provide some additional insight to the equation. A very cold snap which freezes water in the catchment or a dry shoulder season period could drop the hydro generation significantly down, so at that point it would be over to what might be a very minimal solar generation plus the backup generator to handle the input into the system. I suspect we'd adapt to this to some degree - eg using the backup gas oven we can significantly drop the electrical demand by turning off the Everhot - but it will be the heating/hot water which I'd want to be sure will remain sufficient too. How long might a well insulated thermal store retain their heat? That may be part of the answer. However ultimately if we plan for a 3 to 4.5kW hydro feed I understand the thermal store would need to be a very large size to capture all of that dumped electrical generation, and I'm then assuming that may prove problematic as it could be oversized for the solar/backup generator to heat up from the feed they would be able to put in, particularly in an overcast winter or shoulder season.

[Rendall]

8 hours ago, Ed Davies said:

 

Which is cheaper, a spare hydro turbine and misc repair materials for the penstock, etc, or an oil boiler and tank? A spare turbine on a shelf in a dry room needs less servicing than an oil boiler that's not being used much.

 

Good point Ed. The issue would remain that I mentioned to Jamielad - it might not be a fault that means the hydro can't run, but a hard frozen spell or a excessively dry spell. In that case though to extend your logic, it could be that a hotel room for a few days may be cheaper and more enjoyable than putting in an infrequently used oil boiler!

[Rendall]

8 hours ago, SteamyTea said:

Yes, or even Btus, just really confused.

Is a BTU the energy needed to heat a lb of water 1°F?

SI us so much better, but then it is more fun bamboozling people. 

 

Yup, I still confused @SteamyTea...actually maybe more

 

If I follow correctly, you're saying my 88 kBTU calc is the same as a 26kWh demand; ie by that I'll need 26kW for one hour to get it to the right heat level? Or is the BTU calc I did the daily 24hr demand - so the equivalent of 26kWh/day - and hence your comment that with a 3kW hydro, a third of that (ie 1kW/24kWh per day) is the need for space heating?

 

Presumably then though there is the little and often (or indeed perhaps a lot and often) continuing heat to sustain it and mitigate for heat loss?

 

 

[SteamyTea]

If it is a proper 88 kBTU, then that is 26 kWh.

Or just over a constant 1 kW (1.08 kW).

26 kWh / 24 h

 

Get your heating people to give the answers in SI rather than Imperial.

 

(You may find there is a few of us that seem a bit nitpicky about the whole kW and kWh thing, the idea is to save confusion in the long run.  We also like to work in SI units as these are established and agreed upon) 

[Simplysimon]

with hydro you would also need an abstraction license

[Rendall]

1 minute ago, Simplysimon said:

with hydro you would also need an abstraction license

 Yes - we'll apply to Natural Resource Wales for this.