I have a 5kw water heater and a 9.6kw solar system with 8.2kW inverter. I would like to regulate the hot water system so that it takes only the power coming from the solar panels. So if panels are producing 3kW then the heater would only use 3kW. If the panels are producing 8kW then the heater would max out at 5 and I could feed 3kW into the grid or use it for something else. I would like to control this with a raspberry pi or arduino. I have the pi reading from my inverter no problem and it is also measuring the current going to the hot water. I just need a way to regulate the hot water system somehow. Is there some sort of PWM device that can handle 5000W and be controlled by a small voltage input? Also, is it feasible to make this legal? I was thinking an electrician could install the PWM device and I could do the raspberry pi side.

 

Hello,

You'd have to get UL approval to make it perfectly legal. That's expensive.

As far as controlling, maybe a high power solid state relay or a contactor, or some decent transistors.

Is the heater AC or DC?

 

Making it legal would depend on which country you are in, what that country's wiring codes are, and having official approval for any grid feed-in arrangement.
For an AC heater, burst-mode control would probably be used.

 

It's an AC 240V 50Hz heater. The solar system and hot water service are legally installed by professionals. It's just the circuit to regulate it that I would like to add.

 

Maybe I'm missing something here, but assuming the inverter is set to output 250VAC maximum and the heater is driven directly by the inverter, then the heater will draw 5kW maximum even if the panel provides 8kW. If the panel puts out only 3kW then the inverter output voltage will drop and the heater will inevitably draw no more than 3kW. I don't see that any control is needed.

 

I have what your looking for on my OFF GRID system.
Why do you want it for an ON GRID system it goes to the water heater anyway when its calling for heat ?

 

Alec_t, it is connected to the grid. If it heats at 5kW and the solar is producing 3kW then I will be paying for 2kW. But if I heat slower over a longer period of time then I will pay nothing.

 

Wow- that's some expensive hot water!

 

Hook the hot water heater up to 120v then it will heat slower and draw only 1.2 Kw and take 4 times longer to heat the water. If your going to use an inverter use a 120v one and only hook it to the bottom element , turn down the temp on the top element a bit, then the inverter will do most of the work and the top will be sort of a back up for no sun.

If your hooked to the grid it makes no sense to do it though.

Tell us the truth, Your system is not paying off is it. Solar does not pay off it just lets you feel green.
Been doing it for 8 years now and It does NOT pay off. The panels only put out about 50% of what they say.
I have tried 5 different manufactures and none put out what they say.

 

It's about $550AU a year ($350 US?) without the solar. With the solar it's quite cheap considering we pay only a small bill and don't have a gas bill. It could be cheaper again if I time it to the solar generation a bit better.

 

Hook the hot water heater up to 120v then it will heat slower and draw only 1.2 Kw and take 4 times longer to heat the water. If your going to use an inverter use a 120v one and only hook it to the bottom element , turn down the temp on the top element a bit, then the inverter will do most of the work and the top will be sort of a back up for no sun.

If your hooked to the grid it makes no sense to do it though.

Tell us the truth, Your system is not paying off is it. Solar does not pay off it just lets you feel green.
Been doing it for 8 years now and It does NOT pay off. The panels only put out about 50% of what they say.
I have tried 5 different manufactures and none put out what they say.

I can't hook it to 120V as the inverter is connected to mains. AFAIK the hot water only has 1 element. With regards the value of solar, it was $10k installed and is saving around $2500 a year, so will be all paid for in 4 years and then be a bonus after that. We get decent rebates here and I bought pretty much the biggest system I could fit. Also, they have incentives for feed in tariff so we get 12c to feed it in and pay 19c for power from the grid. With regards the panels only putting out half, that's why I got a 10kW system as we can only feed 5kw back to the grid. Although it has most definitely gone to it's limit many times over the summer, putting out 8.2kW. The idea of optimizing the hot water is to reduce the bill down to zero if possible. It is our biggest power usage. It's also really just a bit of a hobby project.

I looked at solar hot water but the payoff is too long unless our hot water packs it in. I also considered batteries but payoff on them is way too long considering the 12c/19c prices are fairly close. It's like using the grid as a battery which is 63% efficient I guess.

 

If the heater is grid-connected then I think the usual burst-mode control and phase-angle control methods to get 3kW heating would still (depending on your existing control system) result in power being taken from the grid, albeit intermittently. Perhaps an auxiliary heater (say 3kW rated) could be switched in series with or in place of the present 5kW one to reduce the load when the solar provides only 3kW. How will you know when that is?

 

Hello again,

Let me restate the problem in a few simple statements.

1. We have a 5000 watt heater connected to the grid.
2. We have a 8kw solar panel that sometimes only puts out 1kw to 3kw.
3. We want to reduce power to the heater when the solar panel is putting out less than 5kw.

For this we first need to monitor the power output of the solar panel so we know when to reduce power and when not to reduce power.

Since the main reason we want to reduce power is to save money on the electric bill, we can either use normal PWM or a very slow low frequency PWM which is really just pulsing the heater on and off at a very low frequency. For example, even if we turn it on for one full minute and off for one full minute we will get a net power into the hater of 2500 watts which is 50 percent of full power. There are many ways to do this either solid state or contactor, but i would prefer solid state like a high power solid state relay which also allows low voltage DC control.

The power from the solar panel is voltage times current, so if we measure voltage and current we know how much power it can supply. If the voltage output is fairly constant then we can measure just current and use that as the reference. When the current is high enough we allow full power, and throttle it back when lower accordingly.

Again as far as getting it legal i dont think that will happen because you have to get approval from your local regulatory commission. Here in the USA we have UL and that is somewhat expensive and if it does not pass you have to pay again for the next test. The best you can do it make it as safe as you possibly can. That's not too hard to do though as long as you use the right gauge wire, fuse everything, make sure everything is grounded properly, etc.

 

Thanks everyone for the replies. I found a device that will do it but I've just moved to a new electricity provider and the amount the pay on the solar is the same as the amount they charge for the hot water off peak. So there's limited point in doing this now. I could time it so it runs only when the solar is outputting above the 5kw that we are allowed to feed in but I think I'll get too small a gain there.

Alec_t with regards your comment about power being taken from the grid in very short bursts I had wondered that myself. It might depend on how high speed the meters are.

http://www.brocott.co.uk/motor-speed-controller-dimmer-240vac-110vac-25-amp/

Thanks again for the replies

 

Who is your new power retailer and which State are you in (in Aus)?

For what it's worth, we have 4kW of grid connected solar power installed about 10 years ago, a thermally very efficient house (owner built mud brick with very good roof insulation), LED only lighting and our two-person household is not particularly frugal with power.
We haven't paid for ANY electricity in about 4 years - since the panels had paid for themselves.
Our summer accounts are always in credit - and our winter bills reduce that to near zero. Then the summer quarter comes again....
Not sure why N11778 would question the value of solar panels.... Why?

 

One last comment - for others contemplating solar hot water:
The heating elements in typical hot water systems don't care if they get AC or DC - nor what voltage it is, as long as it's limited to below the nominal voltage its designed for. So one of the options is to install panels with an output of the rated power of the element, connect them in such a way that their nominal voltage output is at or just below the element rating, and then connect them DIRECTLY to the element. The hot water service's thermostat will simply switch the power off when the water is hot enough and leave the solar panels open circuit. That is a 100% efficient system without any connection to the grid - so it doesn't have to be approved or anything. It is even possible to get32, 42 and 50V heather elements do that an insurance company can't complain if you install the system yourself.....

A slight improvement might to to have the thermostat operate a change over switch instead of just opening the circuit. That way, the solar array could be switched to a grid-connected or battery charger inverter/regulator and feed any exess power to the house or the grid....

 

Who is your new power retailer and which State are you in (in Aus)?

For what it's worth, we have 4kW of grid connected solar power installed about 10 years ago, a thermally very efficient house (owner built mud brick with very good roof insulation), LED only lighting and our two-person household is not particularly frugal with power.
We haven't paid for ANY electricity in about 4 years - since the panels had paid for themselves.
Our summer accounts are always in credit - and our winter bills reduce that to near zero. Then the summer quarter comes again....
Not sure why N11778 would question the value of solar panels.... Why?

It's tango in Victoria. Price is slightly higher for the hot water at 12.9c vs 11.3 for solar. With regards value for solar, prices have come down in recent years considerably and with rebates etc it's a no brainer these days. That wasn't always the case and I think a lot of people are going off the old prices. Batteries are currently not worth owning but like solar panels they will come down. Your system would have been expensive in the day and probably would have taken a long time to break even.

 

One last comment - for others contemplating solar hot water:
The heating elements in typical hot water systems don't care if they get AC or DC - nor what voltage it is, as long as it's limited to below the nominal voltage its designed for. So one of the options is to install panels with an output of the rated power of the element, connect them in such a way that their nominal voltage output is at or just below the element rating, and then connect them DIRECTLY to the element. The hot water service's thermostat will simply switch the power off when the water is hot enough and leave the solar panels open circuit. That is a 100% efficient system without any connection to the grid - so it doesn't have to be approved or anything. It is even possible to get32, 42 and 50V heather elements do that an insurance company can't complain if you install the system yourself.....

A slight improvement might to to have the thermostat operate a change over switch instead of just opening the circuit. That way, the solar array could be switched to a grid-connected or battery charger inverter/regulator and feed any exess power to the house or the grid....

I think I could still make some savings by simply switching the unit on during the day when solar goes above a certain point. If it doesn't reach the desired power during the day then it would heat up at night as usual.

 

It's tango in Victoria. Price is slightly higher for the hot water at 12.9c vs 11.3 for solar. With regards value for solar, prices have come down in recent years considerably and with rebates etc it's a no brainer these days. That wasn't always the case and I think a lot of people are going off the old prices. Batteries are currently not worth owning but like solar panels they will come down. Your system would have been expensive in the day and probably would have taken a long time to break even.

As it turns out, I was an early bird on a community project of "1000 Solar Roofs", which was sponsored by the government at the time. So furtunatelythe then 'normal' prid of $13000 per kW solar panels came down to just under $3000 installed (and bought as a lot of 1000 installations by the local community project management).
It has long paid for itself.
I bought a 5kW system for my daughter for $2900 installed 2 years ago (on Gray's Auctions) - that indicates the changes in costs over the past 10 years here in Vctoria, Australia.
I agree the cost of a battery installation is not yet economically sensible, but coming closer.
Tango Electricity is not available in my geographic area of Victoria, but their plans are very good.
Thanks for pointing them out to me.

 

I think I could still make some savings by simply switching the unit on during the day when solar goes above a certain point. If it doesn't reach the desired power during the day then it would heat up at night as usual.

I agree with you. Soing that manually is easy, but doing it automatically might be tricky, unless you find a 'friendly' electrician who's prepared to sign for the connection of a relay (mech or electronic). I guess your design needs to be fail safe - which is not hard, since the hot water service thermostat will turn off when it's hot anyway....