Battery system for off grid house

ZeroEnergyGC

An off grid home with a 15kW PV system requires a battery system to cover the months of Oct,Nov,Dec,Jan....
3200kWh needed to cover those months.
Can anyone solve this and save me a day researching

freddyuk

How is the 15kwp PV utilised/dumped during the summer? Are you using all the power?
Are we agreed that 3200 kWh is 26 kWh per day? This is quite high if you are looking to go Off Grid.
First task is to reduce daily power loads and then design a system rather than over compensate with excess battery storage which will be expensive.
With 15kwp you should be covering your base load on even the dullest days unless there is water/space heating going on. So battery only required for night time loads.
Please explain more what is running to use 26kWh per day?

ZeroEnergyGC

Yes this house uses quite a large amount of energy daily as its powering every element of the dwelling from electricity ie water/space heating, and is a 1980s retrofit. I am simulating on a Passive house also for comparison which uses dramatically less power. Both are ran on IESve and match with the actual enegry consumptions of the dwellings so i cant alter the figures really. Thus its going to be a large battery system to cover the four winter months when there is alot less Energy being produced. Some energy will indeed be dumped during the summer. Really its the maths involved in sizing the battery system which is causing some confusion.

ted1

ZeroEnergyGC wrote: »

Yes this house uses quite a large amount of energy daily as its powering every element of the dwelling from electricity ie water/space heating, and is a 1980s retrofit. I am simulating on a Passive house also for comparison which uses dramatically less power. Both are ran on IESve and match with the actual enegry consumptions of the dwellings so i cant alter the figures really. Thus its going to be a large battery system to cover the four winter months when there is alot less Energy being produced. Some energy will indeed be dumped during the summer. Really its the maths involved in sizing the battery system which is causing some confusion.

Is this a study or are you actually planning to do it ?
Can I ask what is your objective? Because a diesel generator would be a very good option.

With regards the space and water heating, what type are you using ? Heat pumps ?
What type insulating has the property ? Your money may be better spent on insulation.
With LED and A rated appliances your load should be low. So you should really look at the space heating and why your heating so much.

Perhaps use a solid fuel stove and then you'll have dramatically cut your load

freddyuk

Exactly. Having a generator means the peak loads will be covered without over investing in solar or battery capacity. Getting rid of the high peak loads such as Immersions and electric ovens allows the system to run effectively on solar most of the time and generator kicks in automatically when required to cover excessive loads or to recharge batteries during long dark days. This is cheaper in the long run.
If heat pump is used it can be linked to perform when solar is surplus.
Planning a load profile across the year will allow you to ensure that free power is best utilised and storage is sufficient. It is not an exact science so generator chops off the peaks so you can design nearer to your baseline load number.
Just having a big battery bank is not the answer if you don't have the solar to recharge it. It's a balance.

quentingargan

ZeroEnergyGC wrote: »

Yes this house uses quite a large amount of energy daily as its powering every element of the dwelling from electricity ie water/space heating, and is a 1980s retrofit. I am simulating on a Passive house also for comparison which uses dramatically less power. Both are ran on IESve and match with the actual enegry consumptions of the dwellings so i cant alter the figures really. Thus its going to be a large battery system to cover the four winter months when there is alot less Energy being produced. Some energy will indeed be dumped during the summer. Really its the maths involved in sizing the battery system which is causing some confusion.

Batteries are never used for seasonal storage (from summer to winter for example). 3,000Kwhrs would require 250,000 Ah or about 1,000 250Ah batteries. If not Li-Ion, you would need twice that as lead acid batteries are normally best discharged to 50%

What you need is enough solar to provide your load, with batteries storing for the few dull days in a row you might get. In off-grid calculation, these are called "black day equivalents".

I use Pv-Syst to do simulations of off-grid systems. You need to know the loads, time of loads and then the system will suggest a combination of solar array and batteries to meet that. There is normally a small amount (5% or less) of the load which is met by a diesel backup, but that can be eliminated with more batteries, just not 1,000 of 'em.

I think you need to consider the suggestions to eliminate some of the loads, cooking with gas etc., but if you want to be completely running off sustainable energy, you could incorporate a wind turbine, though I don't normally recommend that. Or if you don't fancy having a turbine, you could also consider occasionally charging your batteries using off-peak power, which often in the middle of the night comes mostly from wind.

ted1

quentingargan wrote:

Or if you don't fancy having a turbine, you could also consider occasionally charging your batteries using off-peak power, which often in the middle of the night comes mostly from wind.

Ironically that's when the grid is at its highest output with regards CO2/kWh as the CCGT plants are off and moneypoint is providing a large percentage

quentingargan

ted1 wrote: »

Ironically that's when the grid is at its highest output with regards CO2/kWh as the CCGT plants are off and moneypoint is providing a large percentage

Yes, I have seen this argument before, but I suspect that the increase in CO2 caused by additional off-grid loads would be lower per KwHr. Also, there is regularly curtailment of wind at night for lack of demand.

ted1

quentingargan wrote: »

Yes, I have seen this argument before, but I suspect that the increase in CO2 caused by additional off-grid loads would be lower per KwHr. Also, there is regularly curtailment of wind at night for lack of demand.

It's not an argument it's a fact. The plants that produce baseliad have the highest CO2, when the cleaner plant is off the Kg CO2/kWh increases significantly, this happens at night.

Wind is curtailed by the restriction on the SNSP limit as a percentage and also the number of thermal units that are operating.

Regardless whatever wind is on the grid is used so from a end user point of view it doesn't matter what time of the day you use it.

quentingargan

ted1 wrote: »

It's not an argument it's a fact. The plants that produce baseliad have the highest CO2, when the cleaner plant is off the Kg CO2/kWh increases significantly, this happens at night.

Wind is curtailed by the restriction on the SNSP limit as a percentage and also the number of thermal units that are operating.

Regardless whatever wind is on the grid is used so from a end user point of view it doesn't matter what time of the day you use it.

Yes, I get that. But I don't think this is a reason not to use power at night. I would need to see what EXTRA CO2 is emitted as a result of increasing the load.

But we're off topic here. I think the OP was looking at a completely off grid situation.