Musing on more PV vs storage

[DamonHD]

Hi,

As a thought experiment I like to consider what would be necessary for us to never import from the grid from time to time.  In sunny suburban London this is wishful thinking and we have reliable supplies, though we are about zero-carbon and seemingly one of the most efficient UK households for energy.

Which grid?  Just electricity for now.  Note that this isn't an effort to save money: the grid is *cheap* compared to anything we could do.

We generate about twice what we use year-round, circa 3.6MWh/y vs 1.8MWh/y.

For all but 4 months of the year we generate more than we consume each day on average.

So a small amount of storage, eg a day or two or three, would mean that we could not import anything from March to October inclusive.  That might cost us at £500--£1000/kWh in 90%+ efficient low-self-discharge Li batteries maybe £10k.

How to ride out Nov--Feb?  At worst our PV should produce about half what we use each day on average, so extra storage for the balance of maybe 180kWh might cost ~£100k and then we need never import again.  (All to save about £20-worth of imported energy!)

Alternatively we could in principle put up about another 2.5kWp of south-facing 60-degree-tilted panels if we turned over our small garden entirely to the cause to boost mid-winter generation by the short-fall.  That would be £10k.

So maybe we could come off-grid (except to export for income) for £10k of storage and £10k of extra panels.

But what if we wanted to come off the gas grid too, for heating (water and space).  Gas consumption last year was a little over 6MWh.  Assuming for the moment that we could get a CoP on a heat-pump of 3 year-round, 2 in winter, we'd need 2MWh, amounting to an extra 20kWh/d of electricity mid-winter for heating, implying £1m in storage of £100k of south-facing panels (though only by taking over about 10 neighbouring gardens), or a heck of a lot more insulation!

So, coming off the gas grid in this house looks implausible so far due to the laws of physics and my bank account...

Rgds

Damon

[TomW]

Yeah "off Grid" flammable gas would be a challenge unless you have a large herd of bovines stashed in the garden ;=>

Enough for cooking would be semi trivial but heating is another animal.

Heard London called many things but never "sunny".

Maybe you could collect the smog and condense out flammable liquids? Then build a still to produce the lighter hydrocarbons?

Half joking half "I wonder ..."

We burn zero carbon  [short cycle anyway] wood and I don't know how I would do it if that option was not available.

Lots of south facing glass works for a good bit of the year but only a small % here.

Good luck on the gas side of it it is a real skull buster.

If you get really big on storage and solar mayhap you could run a heat pump? Just tossing stuff on the wall here to see what sticks.


Tom

[DamonHD]

Our run of terrace houses is south of us and our walls with windows face east and west (north is a blank wall).

I'm already making use of most of our roof-space with PV, and some of our garden space with off-grid PV for some limited south-facing exposure.

I do see a heat-pump in our future, when the market is more developed and the "ECO-CUTE" R744 (CO2) based heat-pumps that I prefer the look of (can deliver water at over 60C without loss of performance) are more widely available and are available in 'tankless'/'instant' hot-water variants.  (Hitachi had one here, but withdrew it.)

Rgds

Damon

[rossw]

But what if we wanted to come off the gas grid too, for heating (water and space).  Gas consumption last year was a little over 6MWh.  Assuming for the moment that we could get a CoP on a heat-pump of 3 year-round, 2 in winter, we'd need 2MWh, amounting to an extra 20kWh/d of electricity mid-winter for heating, implying £1m in storage of £100k of south-facing panels (though only by taking over about 10 neighbouring gardens), or a heck of a lot more insulation!

So, coming off the gas grid in this house looks implausible so far due to the laws of physics and my bank account...

Your calculations seem to be based entirely on PV. Conversion of sunlight to electricity is around 15%, but lets be generous and say 20%. Putting that into batteries to store, and later producing heat using heat-pump, even with CoP of 2, we're talking about a maximum of 40% equivalent sun-to-heat.

Using solar thermal collectors, you can achieve 90% odd conversion. Lets be overly harsh and say 80%. That's still TWICE the conversion efficiency of using PV. You don't need the expensive batteries, you don't need the expensive PV modules. Your storage medium can be as simple as a large water storage tank, or you could put hydronics pipes down and pour a concrete slurry over your floors and use that as active radiant slab and combined heat storage.

Just something to consider....

[DamonHD]

Hi,

My understanding is that solar thermal is more typically 50% efficient when all is said and done, and a CoP of ~2.5 with the panels I'm using would match that in overall energy capture.

What we lack is space, and the remaining place we had to put solar thermal was directly above my daughter's bedroom.  Given a very nasty accident still vivid in my mind where a baby was scalded to death with a faulty hot-water system directly over its cot, I didn't want to go that route.  And excess hot water in summer can't be exported to the grid and help reduce carbon emissions elsewhere.  Plus we're also currently instant/tankless and I don't want to re-introduce those storage losses into my system.  (And the space where the tank was is in use...)

I have thoroughly considered thermal, and will revisit it if I think it will give any significant gains for us over PV.

Rgds

Damon

[hydrosun]

Adding a small solar water heater sized just for hot water use would displace the gas used in the summer and when the sun is shining. Your gas heater isn't 100% efficient so the replacement heat needed is less. The chimney of the gas heater is pulling air through your house unless you have an outside source of air. An air to air heat exchanger and a better sealed house would further cut down the heat needed
. We heat our 1000 square foot house with a 9000 watt ductless heat pump. If our hydro is going full out we can heat the house with the heat pump and have enough extra power to heat the water tank too. If we don't have enough power I just start the wood stove for heat and hot water.   I've been investigating the inverter heat pumps made by Sanyo that can work down to 200 watts input. At 47F it has a COP of 5 at this lower output. Full output is still at COP of 3. Our hydro output goes up and down all winter and the heat pump is turned on and off by the voltage control of the Outback inverter. It would be nice to just lower the power usage to match the surplus and have the bonus of higher efficiency. I read a discussion on the wind-sun forum about several off gridders using them for low output airconditioning in California summertime. Only drawback was the need to manually change the power usage to match the power available. If you are home you could increase the speed when the solar panels were putting out more power.  The 22 SEER 9000 btu ductless heat pump is on ebay for $1024 plus shipping in the US. On their website the shipping is free.  If we didn't already have a heat pump I would buy this one. But I'm hesitant to pull out this one to put in one a bit more efficient.

[DamonHD]

A Sanyo ASHP is the one I'm interested in, indeed, but in the UK at least there is no tankless/instant variant.

The manufacturer of our existing gas boiler says that we cannot pre-heat water to it (though we could of course mix down solar-heated water to 25C to feed in) so adding any solar thermal would be a fairly large upheaval and I'd rather wait until/if we replace the whole lot and remove the dependence on FF entirely.

Rgds

Damon

[PaulJ]

"Alternatively we could in principle put up about another 2.5kWp of south-facing 60-degree-tilted panels if we turned over our small garden entirely to the cause to boost mid-winter generation by the short-fall.  That would be £10k.

So maybe we could come off-grid (except to export for income) for £10k of storage and £10k of extra panels."

Good quality panels might be making a good percentage of their rated power in a century.

Good quality batteries would be lucky to make 15 years before heading off for recycling (hopefully).

I think you're on the right track with the extra panels rather than masses of batteries for long term emission reduction providing your summer excess is sold back to the grid. Makes no economic sense, though I realize this is not the point.

   Paul.