More diversion load stuff

[Joe_B]

Hi guys,

The recent "what do I do with excess energy" thread here got me thinking, I have a 40 gallon LPG water heater that is killing me on propane bills. My PV system produces extra power and usually once my batteries are charged, my controller goes into float mode and unloads the panels. I have an XW system with 2 XWSCC's.

So I am thinking of installing a water preheater tank as a dump load and have a few questions.

1. I am planning on using a large solid state relay to switch my 24V battery system to the water heater element. I am using the XWSCC aux output to control the relay. I will set the aux output to connect the battery when the controller reaches the float voltage. Anyone see any issues with this?

2. I was looking at 24 volt heater elements and they go for anywhere from $30 to over $100. The heater I am looking at installing is a 20 gallon, 110V, 2KW. The element I was looking at runs on 24V at 20A for 480W. Here is the question, If I hook 24 volts to the existing 110V/2KW element, it should run at one quarter of the power or 500W. Should I just buy a 24V element or hook 24VDC to the original equipment?

The heater will be within 5 feet (wire distance) of the battery bank, I am planning to use a 60 amp SSR (Crydom D06D60) and 4 ga welding cable to hook things up, the SSR will live in the XWPDP.

So what do you think? Am I doing anything stupid? I can get the water heater from HD for about $270, the relay goes for about $100 I have all the rest of the stuff except for the 24V element if i have to buy one. so for less than $400 my numbers say this should be worth it considering I am burning about $100 in propane each month.

[BB.]

How much excess worth of power do you have per day/month (kWH?).

It would seem if you do not have much power, then the costs would not make it worth the money.

If you have a lot of power, then looking at heat pump type water heater would then be interesting (roughly 2x more heat from same input energy as a resistance heater).

Other way would be a thermal solar array and run the circulation pump with your current DC battery bank (although, a nominal solar PV panels connected to a DC circulation pump would work too.

Back to your setup--you have a fuse/breaker on that line? Resistance heaters in hot water tanks can fail with a dead short.

Also, you should not need to limit yourself to DC rated elements... Get a much larger AC element and derate the wattage by:

• Element Watts * (Vbatt)^2 / (Velement)^2

You may save some money.

-Bill

[Joe_B]

Thanks for the information Bill, I currently have six SW235's on the roof (1 SCC) and they are producing about 6.5 KWh per day in full sun with this daylight time. I expect to get about 8 KWh or so at peak summer time. I have 840 AH of trojan L16REB batteries at 24V.

My issue is this, I do not have sell enabled and I do run grid support. If I do not switch the grid support voltage setting depending on sunny or cloudy days, I lose a lot. I run the house in grid support mode at night so the batteries ramp down. During the day, the batteries get charged and then the controller loafs at the house load, I can see that I am wasting about 750 watts when this happens.

So what I do is if it looks like it is going to be cloudy, I set my grid support voltage to 32V which is Xantrex code for "only go into grid support when the batteries are at 100%". In thiss mode, my batteries are fully charged and then the house takes the load. If it looks like it will be sunny, I set the grid support volts to 24.6V and let the batteries discharge at night and recharge during the day, obviously harder on the batteries.

So what I would like to do is leave grid support volts set to 32V which will essentially put my batteries in a float mode but during sunny days when the power would normally be wasted I will dump the energy into the water thereby hopefully lowering my propane demands.

As for the wiring, I have will use a 75 amp DC breaker.

Thanks, I thought resistors are resistors but just needed to hear it about the element. You just saved me some money. I am assuming that running DC through the OEM thermostat is a bad thing, right?

[Ralph Day]

I have a SSR running off my MX60. The relay energizes a regular house outlet powering an oil filled electric heater, or dehumidifier, or distiller.

Why not just run the water heater as bought...120vac. The SSR will turn on and off when needed (very fast too) using inverter power. You can have a hold or delay on the aux energizing too. This would be the cheapest way and avoid low voltage heating element. You could always go that way if you found that the 120vac way didn't work.

Ralph

[Joe_B]

I have a SSR running off my MX60. The relay energizes a regular house outlet powering an oil filled electric heater, or dehumidifier, or distiller.

Why not just run the water heater as bought...120vac. The SSR will turn on and off when needed (very fast too) using inverter power. You can have a hold or delay on the aux energizing too. This would be the cheapest way and avoid low voltage heating element. You could always go that way if you found that the 120vac way didn't work.

Ralph

Hi Ralph,

I do not want to load my inverter if I dont have to. With an extra 2KW load on the inverter, I could overload things if I turn on other stuff. The XW4024 is only good for 4KW. Since the heater is so close to the batteries, its more efficient to do it with DC. Also seems like I can just run the 2KW OEM element at 24 volts and get about 500 watts of load. Win-win!

[BB.]

I am wondering if running a straight AC water heater would be better than trying to run a 24 volt conversion--You are only loosing the 5-10% of the XW inverter losses.

And, running the AC contactor on DC can be an issue... It is probably not designed or rated for DC--but you may be running substantially less DC current--so it is possible that it could work out OK (welded/arcing contacts being the big issue).

A $100 gallons of propane per month towards what? Cooking, domestic hot water, domestic heating?

I am guessing that you are probably paying 2x or more for the same "btu" content as I do for natural gas... I run around $20-$60 per month (summer/winter) to heat, cook, hot water, cloths washing/drying (clothes drier in winter).

Assuming 750 watts * 6 hours a day of lost power = 4.5 kWH per day "lost"...

That would be around:
4.5kW per day * 3,412.3 BTU/kWH = 15,355 BTU's per day "worth of electricity"

A gallon of propane contains around 91,600 BTU per gallon... 30 days of 100% electrical conversion:

• 15,335 BTU per day * 30 days / 91,600 BTU per G = 5 gallons per month (towards resistance heating)

So, 5 gallons per month propane savings with a resistance based electric water heater (I assume that I understood your numbers correctly).

If you got a Heat Pump type water heater, that would be around 10 gallons per month savings...

If I got the numbers wrong--you can "adjust" them to fit your needs.

In the end, try to estimate where your fuel costs are going (hot water, cooking, washing clothes, space heating, etc.) and run the figures one what would make sense... Conservation (insulation, double pane windows, low flow shower heads, use thermos cooking for stews and soups, etc.) vs where you would like to spend your money on the technical side (solar thermal vs heat pump water heater, etc.).

Is it worth it to you?

-Bill

[Joe_B]

Well Bill, if you are going to apply logic and math to this.....

Seriously, I am using propane pretty much exclusively for DHW. I have a cook top but it hardly ever gets used.

Now to the numbers:

I am presently using about 40 gallons of propane every 2 months and paying $5.96 per gal. Last bill was $209.00 for 2 months.

So if I use your numbers and not factoring in conversion losses, I am looking at saving $29.75 each month. With this information, the install will pay for itself in a little over a year. Seems like a reasonable payback.

The reason I do not have a heat pump water heater is that I dont want to load my inverter with it. I only have 4KW available and sometimes I get close, so I have propane with pilot, no electrical load.

[BB.]

It appears you can set the GE GeoSpring to Heat Pump Mode only, which:

How long should my GeoSpring water heater run in order to reheat the water in the water heater?

• The recovery time is a function of a number of different factors including the amount of hot water used, the temperature of the incoming water, the ambient air temperature, heating source, etc. The GeoSpring heat pump can recover 8 gallons per hour (at 68°F ambient air temp) while standard heating elements can recover about 22 gallons per hour. The difference is, the heat pump only draws about 550 watts of energy compared to 4,500 watts from standard heating elements. So if an average shower uses 16 gallons of hot water, the heat pump would run about 2 hours to reheat the water and use about 1.2 kilowatt hours of electricity. For the same 16 gallons of water, a standard electric water heater would take about 48 minutes at 4,500 watts of power which is about 3.2 kilowatt hours, almost three times the energy. Therefore, although the heat pump runs longer to reheat the water, it still ends up using approximately 62% less energy. Since most consumers use hot water in varying amounts followed by long periods of little or no usage, the heat pump has more than sufficient time to reheat the water and satisfy the consumer’s hot water demands. If more than the average amount of water is used, GeoSpring is uniquely designed to switch from heat pump to resistance heat and back again, to provide the same volume of hot water as a standard electric water heater. In the winter time when ambient air temperatures are likely to be lower, the heat pump will have to run longer to heat the water.

I don't think you are "wasting" Propane... At $6.00 per gallon (eek!), I would be looking for alternative fuels (diesel, gasoline, etc.)... Propane has a lot less heat per gallon than either of those other options.

you are paying:

• $6/0.0916 MBTU = $65.50 per MBTU
• Natural Gas ~ $12 or so per MBTU (10xTherm) (PG&E)
• Diesel ~ $4.50 per gallon / 0.135 MBTU per gallon = $33.00 per MBTU

And, I would seriously look at Solar Thermal panels for DHW and Space Heating--If you are into plumbing DIY--Not a bad alternative.

Solar Shed and other Solar Thermal Links

-Bill

[Joe_B]

Yeah Bill,
Geospring $1500, crappy little preheat tank $268
Cant deal with it.

As far as the DC on the thermostat, I was thinking of putting a snubber cap across the contacts just for the heck of it.

[BB.]

I think somebody around here got one for under $1,000 with Sears (?) and the federal tax credit...

Use this fuel cost calculator and plug in $5.95 per gallon for propane and $0.13 per kWH for electricity (or whatever you are using):

$38.09 per Million BTU of Heat delivered to home
$3,618.55 per year for normal home for Electric

$98.90 per Million BTU of Heat delivered to home
$9,395.50 per year for normal home for LP (Propane) Gas

Add in that the heat pump hot water is 2x the efficiency of electric resistance heaters (assuming ~>55F ambient temperatures), you would get ~5x the amount of hot water with electricity as you do with traditional electrical resistance heaters.

Even if electricity is 2x, you still come out way ahead (then have to look at the payback time on the expensive hot water heat pump system).

That propane is scary costly.

-Bill