Heating a tiny shed

[OldMcDonald]

I have a wooden garden shed about 6m x 4m with walls and ceiling reasonably well insulated. I use it for several purposes – seed house, vegetable production, 8 stacking wormeries, wine cellar and some general storage. It is quite full, so little empty air space, or spare floor space.

I presently use a permanently switched on 1.5kW fan heater at floor level on a very low setting and it is sufficient to maintain 14º to 15ºC (just below 60F) most of the time. It operates only infrequently during the day. The temperature does drop on cold nights (only very slight frosts now and again), but a max/min shows it has not gone below 12.3º in over a year at distance from the heater. That is acceptable for everything in there. Daytime requirements will be less than the 1.5kW.

I am in the process of switching electricity tariffs so that I have a reduced rate from 00.30 to 7.30 a.m. My idea is to continue using the fan heater during the lower rate electricity time as well as making use of this time for my LED grow lights. I want the fan in use because it blows warmer air around the wormeries when on and this raises their temperature above the ambient air temperature in the shed and they are doing OK.

I wondered whether it would be feasible to charge a battery for the 7hrs when electric costs are lower, then use that stored power for an oil filled radiator even for part of the remaining time. This is the only question I want to be answered, and I ask it because I have no experience of using battery storage. Just think cents instead of pence, and the prices are 16p and 33p/kWh. I would need to buy an inverter and battery, but have a radiator that I could use.

I have posted the foregoing information on dedicated sites, but nobody has answered the question, they just want to tell me all sorts of things I did not ask and make suggestions for alternative heating methods – all of which I either cannot have or do not want. If it is not feasible then I will just stay with what I use at present and experiment with the radiator on a low setting and time switch for during the day.

Please do not waste your time on giving me other ideas. What I have and do does not warrant capital expenditure. The shed is organised in such a way that fixed storage heaters, fires, stoves, heat pumps etc. cannot be used. It needs to be small and mobile, hence the radiator idea.

 

[34Willys]

I have read where people use the TESLA power wall, grid tied, and charge it over night when the rates are low, then switch over to the battery when rates are high. These are wealthy people, so...
The answer is yes, but you need to do the math. I don't know the formula, but power consumption by the radiator, storage size and type of batteries, inefficiency of inverter and converter to charge....
ROI may take a lifetime.
Batteries and inverters are pricey.
We are currently off grid, so dealing with similar issues constantly.
Sorry not much specifics.
Patrick

 

[ponytug]

We have AC batteries, and for heating I would not recommend it. Resistance heating takes a great deal of power. I would consider whether there is something that is in the shed that could soak up the heat overnight and radiate it during the day. We once did something similar with 55gallon drums in a greenhouse to keep it warm overnight. Could you put five or ten gallons of water on top of the radiator? A couple of cement blocks?

Given the small cost difference in the rate, I rather doubt that there is going to be a solution that pencils out if you need to buy anything, but perhaps a rescued immersion heater and drum from a skip...

All the best,

Peter

 

[LouNY]

Heat mass storage would likely be your best bet.
As an example a group 31 Diehard Marine Platinum battery at well over $200 is rated at 885 CCA, 1000 CA,
with a 200 minute reserve which ends up being a 100Ah battery.
So, it ends up to this;
1200 watts hours,

If you need to convert amp hours to watt hours, you can do this by multiplying the battery's amp hours by the battery's voltage. Using that equation, the number of watts a 12 volt 100Ah battery can provide in an hour, would be calculated like this: 100 amp hours x 12V = 1200 watts hours or 1200 watts for one hour.

So not much when used on a resistance heater.

 

[Rockbadchild]

sorry but I have to... I am too curious of your reasoning... why not just have the oil radiator instead of both system? what disadvantage you see with the oil system because if it's just for the fan you could have just a fan on batteries for the peak hours that take minimum power and there is oil radiator that don't need any power.

 

[HawkinsHollow]

It is worth a shot!
I think if you made sure you used the cheap electricity to get the radiator up to temp I could see the battery lasting a while. Even better, if you could figure out a way to heat it up to MAX temp using the cheap electricity then turn it to low for the rest of the day it could last even longer. There is a way to figure out all of this info. but you might need a meter like a kill-a-watt or equivalent to determine power draw over the day then scale your batteries up accordingly. But honestly, you have to wonder would the investment in batteries, inverters, timers, etc. just make it as cost efficient to pay the man for the pricier electricity. Good luck, let us know how it goes.

 

[MAX-24-Dean]

Daytime requirements will be less than the 1.5kW.

I wondered whether it would be feasible to charge a battery for the 7hrs when electric costs are lower, then use that stored power for an oil filled radiator even for part of the remaining time. This is the only question I want to be answered, and I ask it because I have no experience of using battery storage.

Probably not feasible? Unless you have money to burn.

Battery Amp draw. Copper cable sizes (costs) are determined based on having enough ampacity to serve the load.
1500 watts / 12 volts = 125 Amps
1500 watts / 24 volts = 62.5 Amps
1500 watts / 48 volts = 31.25 Amps

You would need to provide more information to determine actual sytem requirements.
It would be helpful to know the total Kw hour usage just to have a starting point.

If the heater used the 1500 watts at a 25% on time over the 7 hours. It would look like this.
7 hours * 1500 watts * .25 = 2625 watt hours or 2.625Kwh

So lets use 3Kwh (3000Wh) per day to size a battery.
3000Wh * 1/.90 (inverter efficiency losses) * 1/24 Volt battery * 1/.50 (maximum battery discharge)
3000Wh * 1.11111 * .041666667 * 2 = 277 Amp hour Battery Minimum.
At a 50 % on time, 6000Wh the minimum Amp hour capacity would need to be approximately 555 Amp hours.
Hope this gives you an idea as a starting point for cost approximations.

Edit: misread your post about the 7 hours. I was thinking 7 hours of needing to run the heater intermittently. But you can plug in the actual watt hours used and figure out battery sizing.

 

[OldMcDonald]

After 17 years a member of TBN you would think I should have learned to ask here first!!!!

Thanks for the responses so far. I have skimmed through them because it is getting late here and will give it more thinking time later, but I am fairly well committed for the next couple of days.

I will be back though, and thanks again.

 

[Lastkidpicked]

After 17 years a member of TBN you would think I should have learned to ask here first!!!!

I agree that there are many knowledgeable folks here, and very generous with their knowledge.

If it will save you some time, you can use my experience so you don't have to repeat it on your own.

We have a shed that is too far away from shore power to "plug it in" and are always looking for better ways to provide heat. We always come back to the same conclusion. Nothing we try is as cheap or efficient as simply plugging it into house power, and that is exactly what we would do if we could.

If you have a curious mind, and are looking at this as an engineering challenge, then go for it! But if you are simply interested in the cheapest way to heat your shed, you are probably already doing it.

 

[Egon]

Heat sink storage with the fan for circulation. No batteries as their cost would exceed that of the small circulatory fan.

There should be current system set ups that could be looked at.

Electric Thermal Storage

This may give some information of heat storage systems.