PV solar panels for water heating
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gactrx
Solar Expert Posts: 72 ✭✭
Hi, I'm planning to use 4 x 300 watt solar panels to heat our water in summer. We have no grid power. I think a MPPT controller will help get the most from the panels.
I am thinking if the panels supplying a single battery which will be charged all the time so the controller will continually dump power to the hot water element. Once the hot water is up to temperature the thermostat will close a DC contactor and connect our 880 A/H battery bank onto the controller output,
Has anyone tried this?
Thanks,
gactrx
I am thinking if the panels supplying a single battery which will be charged all the time so the controller will continually dump power to the hot water element. Once the hot water is up to temperature the thermostat will close a DC contactor and connect our 880 A/H battery bank onto the controller output,
Has anyone tried this?
Thanks,
gactrx
Comments
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Re: PV solar panels for water heating
In general, using solar PV panels and battery bank to directly heat water through a standard resistance heater is pretty expensive.
You could do solar thermal panels:Solar Thermal can be a nice source for space heating and hot water... And usually is "cheaper" per kWhr/BTU vs Solar PV Electric. Also, Solar Thermal lends itself very well to do it yourself projects. Note, these are plumbing projects and have their own issues (leaks, pump failures, installation issues trapping air, anti-freeze, storage, heat exchangers, etc.):
Solar Shed and other Solar Thermal Links
A good place to start reading is Home Power Magazine... They have a free past issue online--and have a lot of articles you can read for free. I don't always agree with them and their reviews--but they are a fun and enlightening read:
Home Power Mag
Today--The air to water heat pump water heating systems are getting pretty good. They are about 2-3x more heat per kWH than resistive heaters (as long as ambient is mid 50's or above for many systems). And they output cool/dry air (great for hot/humid environments).
http://basc.pnnl.gov/sites/default/files/case-studies/BA_CS_HPWH_080712.pdf
There are several types, first are standalone water heater+tank:
[h=3]GeoSpring Hybrid-Electric Heat-Pump Hot Water Heater [/h]Another is a retrofit to an existing water heater:
[h=3]Residential | Nyle Water Heating[/h]Anyway--I have not used any of the above--Just starting points for your research. We have some some threads about a few different installations here.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: PV solar panels for water heating
Hi, thanks for your reply. Those heat pumps seem to be grid connected and as mentioned we don't have this. The nearest power is 5 km away and it would be cheaper to move the house to it than get lines put in.
I want to use PV panels because in winter and the cooler months we will use a wood fired water heater to heat the water. A thermal panel, pump, plumbing etc will be sitting doing not much which is a $6000 expensive to be used in summer. At least with PV panels they can supplement our battery's when the days are a bit dim.
Also although there are ways to combat freezing and overheating, they complicate the system.
Cheers, -
Re: PV solar panels for water heatingHi, I'm planning to use 4 x 300 watt solar panels to heat our water in summer. We have no grid power. I think a MPPT controller will help get the most from the panels.
I am thinking if the panels supplying a single battery which will be charged all the time so the controller will continually dump power to the hot water element. Once the hot water is up to temperature the thermostat will close a DC contactor and connect our 880 A/H battery bank onto the controller output,
Has anyone tried this?
Welcome to the forum. Quite a few have tried this, there are threads here and over at the Midnite forum on the subject. The midnite Classic has several features that make it a good choice for your application. That said, my recollection of the discussions is that DC is hard to work with... difficult to switch under load, possibly(?) more dangerous than AC around plumbing, and difficult to transmit over distance... How far will your water tank be from the controller?
Also, why do you want to connect these other panels to your main battery bank? I imagine that if there is enough solar to heat your water, by the time the water is hot your main solar panels will have already charged your main battery bank.
--vtMaps4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i -
Re: PV solar panels for water heating
Hi thanks for your comments. I'll have a look at the Midnite forum for further information. The controller will be right next to the cylinder. I haven't bought and installed the gear yet. Just waiting to work out what to get. I can get suitable DC contactors without much problem so not concerned about that. Although I'm an electrician (since 1977) solar controllers and PV systems are a bit new to me. In regard to connecting the 4 x 300 watt panels, in winter my 2 x 75 watt panels might struggle a bit as I'm hoping to run a small 12/24 volt freezer.
Cheers, -
Re: PV solar panels for water heating
Two words instantly come to mind when I hear of using PV for water heating. 1) Extremely; and 2) inefficient.
I fear you will be hugely disappointed with the amount of warm water produced, and there will be no need for a thermostat, as unless the tank/cylinder holding the water to be heated is very small, the water will never get hot enough to trigger it.
Trying to supply domestic hot water using PV, is possibly the most inefficient of all systems. Directly heating water using a relatively small thermal panel will be far cheaper and far, far more efficient. $6000.00 for a thermal system? Wow! It must be one huge system! -
Re: PV solar panels for water heating
The heat pump systems work with 120/240 VAC--So you would use an AC inverter to do this. Because the heat pump systems deliver 2-3x more hot water per Watt*Hour--The extra costs of an AC inverter is not necessarily a problem.
We can work backwards... How much hot water do you want per day? What is the water temperature coming in?
Guessing in you are around Auckland New Zealand, and using http://solarview.niwa.co.nz/ for information, you get around 2.52 to 5.06 hours of sun per day. Using standard numbers for an off grid system:- 1,200 watt of panels * 0.52 overall AC off grid system efficiency * 4 hours of sun (minimum nominal) per day = 2,496 Watt*Hours per average day ~9 months a year.
Assuming aveage ground water temperature of ~13C and you want 50C minimum water temperature:- 1 liter of water * 1C rise = 1 kCal
- 1 kCal = 0.00116222222 kilowatt*hours
- 2.496 kWH * 1 liter per C * 1/37C rise * 1/0.00116 kWH per kCal = 58 liters per day (using 1,200 watts of solar panel on battery bank using resistance heating)
In the US, solar water heating calculations assume around 40 gallons (151 liters) per day of hot water per person.
If you use a solar thermal collector, in the US, use around 40-80 sqft (3.72-7.43 sqmtr) per 40 gallons of hot water (151 liters).
Agree that freezing temperatures complicate the use of solar thermal... It can be done. Depending on where you are located (amount of sun, temperatures), and how much wood/alternative fuel you wish to use--Many people just drain and shutdown the solar heater for a few months a year.
By the way, be really careful with wood fired boilers for hot water. Many stove and other do it yourself (and even commercial) wood fired water heating systems can have terrible steam explosions.
Heating Water With A Stove - Woodheat.org
There can be other issues with water heating. Do your research (if you have not already).
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: PV solar panels for water heatingThe heat pump systems work with 120/240 VAC--So you would use an AC inverter to do this. Because the heat pump systems deliver 2-3x more hot water per Watt*Hour--The extra costs of an AC inverter is not necessarily a problem.
We can work backwards... How much hot water do you want per day? What is the water temperature coming in?
Guessing in you are around Auckland New Zealand, and using http://solarview.niwa.co.nz/ for information, you get around 2.52 to 5.06 hours of sun per day. Using standard numbers for an off grid system:- 1,200 watt of panels * 0.52 overall AC off grid system efficiency * 4 hours of sun (minimum nominal) per day = 2,496 Watt*Hours per average day ~9 months a year.
- 1 liter of water * 1C rise = 1 kCal
- 1 kCal = 0.00116222222 kilowatt*hours
- 2.496 kWH * 1 liter per C * 1/37C rise * 1/0.00116 kWH per kCal = 58 liters per day (using 1,200 watts of solar panel on battery bank using resistance heating)
If you use a solar thermal collector, in the US, use around 40-80 sqft (3.72-7.43 sqmtr) per 40 gallons of hot water (151 liters).
Agree that freezing temperatures complicate the use of solar thermal... It can be done. Depending on where you are located (amount of sun, temperatures), and how much wood/alternative fuel you wish to use--Many people just drain and shutdown the solar heater for a few months a year.
By the way, be really careful with wood fired boilers for hot water. Many stove and other do it yourself (and even commercial) wood fired water heating systems can have terrible steam explosions.
Heating Water With A Stove - Woodheat.org
There can be other issues with water heating. Do your research (if you have not already).
-Bill
Adding to what Bill wrote, there is a new solution that our friends at http://www.butlersunsolutions.com/node/77 are patenting.
It will be available next month and I am going to use one for a pesky customer in the Colorado.
Basically you use a 1.5KW solar electric array to drive a DC controller that drives an Element that goes into an existing tank (gas or electric).
No pumps, no freeze issues, gas back-up for offgrid, no plumbing - say that again, no heat exchangers, and set it forget it! Listed and safe!"we go where power lines don't" Sierra Nevada mountain area
htps://offgridsolar1.com/
E-mail offgridsolar@sti.net -
Re: PV solar panels for water heatingwaynefromnscanada wrote: »Two words instantly come to mind when I hear of using PV for water heating. 1) Extremely; and 2) inefficient.
Agreed.I fear you will be hugely disappointed with the amount of warm water produced, and there will be no need for a thermostat, as unless the tank/cylinder holding the water to be heated is very small, the water will never get hot enough to trigger it.
Trying to supply domestic hot water using PV, is possibly the most inefficient of all systems. Directly heating water using a relatively small thermal panel will be far cheaper and far, far more efficient.
Nah cant agree with that. We know lots of people that heat most of their hot water with surplus PV. Granted these systems are on the larger side, but the local PV installers here are lately describing the current PV prices as "solar hot water system killers". The real beauty, other than substantially reduced plumbing/frost issues, is matching an oversized array (for generator avoidance) to hot water heating. My modelling shows that even with our smaller 1.8kW array we can heat 75% of our hot water, using just surplus power.$6000.00 for a thermal system? Wow! It must be one huge system!
Well theres hot water thermal systems and hot water thermal systems. A top of the range brand name system with stainless steel mains rated tanks and what not, will set you back around 5K not installed. Even so in temperate climates they have had lots of issues with failed frost valves. There are plenty of cheaper low pressure units, evac tube type systems, that run closed to 2K, and some of them appear to run well.
Heat pump units have moving parts, and so back to maintenance and whatnot.
And as you say solar thermal is definately more efficient, but the question boils down to is the extra 2K plus installation, plus maintenance, better value than adding a few extra panels to your existing array, and just a couple relays to make it go?1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar -
Re: PV solar panels for water heating
I forgot to add, the Butler system cost is $4,500 with the PV and does not include the 30% federal tax credit. Nice safe innovation for an age old problem of contaminating the water system with anti freeze."we go where power lines don't" Sierra Nevada mountain area
htps://offgridsolar1.com/
E-mail offgridsolar@sti.net -
Re: PV solar panels for water heating
My solar thermal system was about $4000 installed, but there is no antifreeze loop. It included permitting, 80 gallons of storage, a solar thermal collector, a circulating pump and Goldline controller. All the plumbing is in copper and has the proper shutoff and drain back for maintenance. The collector has the needed pop off for freeze control in the rare event that it gets that cold here. -
Re: PV solar panels for water heating
Hi, thanks BB for the calculations which I can apply to my system. There certainly is a lot of debate as to the practicality of using PV panels. In my case I want a simple and reliable system and as low tech as possible. Saying that, I'm happy to use a controller to obtain as much performance as I can from the panels. I have emailed for more info on the Butler site. I wouldn't mind going for 5 or 6 300 watt panels if necessary but will start at 4 and add if needed. The cost is quite reasonable now. We don't use a lot of hot water as we are on rain water only. I plan to use a 200lt tank to keep losses low.
I would predict in the future thermal panel use given way to PV as the price drops and they become more efficient.
The thermal system I was looking at does use a stainless 300lt tank and Apricus vacuum tubes, 12v pump and fitting.
Cheers -
Re: PV solar panels for water heating
A simple flat flat hot water collector system, circulating into a tank, coupled with a demand gas unit to supliment is very easy, very cheap to build. I hve built several from virtual scrap (flat steel plate, recycled patio door glass (often free) a simple Delta T controller and a smal circ pump. total cost, under $300 including the copper pipe. Maybe a thousand if you have to pay full retail. I have built several that are are about 35 SW ft, and will easily supply 80 gallons of hot water with a Delta of close to 90F.
A small demand water heater can that uses no elecricity can b had for under $200 for an out door unit, so no venting or install issues. If y don't need freeze protection you are golden!
As has been noted, Using apv to resistance heat water is incredibly inefficient and ergo, incredibly expensive.
Consider this, an electric water heater uses ~ 5.5 kwh to heat 40 gallons. To heat that water from PV you are going to need to create 5.5 kwh of power, which is going to to need an array of perhaps 2 kw to do so (assuming you have ideal solar), and that system, if you include batteries might run $16k.
One more note, solar hot water systems, even for plate systems are much more forgiving of partial shade, or grey sky's. Try this trick, take 100'of garden hose, leave it in the sun for an hour and see how hot it is.
Good luck and keep in touchy,
Tony -
Re: PV solar panels for water heatingHi, thanks BB for the calculations which I can apply to my system. There certainly is a lot of debate as to the practicality of using PV panels. In my case I want a simple and reliable system and as low tech as possible. Saying that, I'm happy to use a controller to obtain as much performance as I can from the panels. I have emailed for more info on the Butler site. I wouldn't mind going for 5 or 6 300 watt panels if necessary but will start at 4 and add if needed. The cost is quite reasonable now. We don't use a lot of hot water as we are on rain water only. I plan to use a 200lt tank to keep losses low.
I would predict in the future thermal panel use given way to PV as the price drops and they become more efficient.
The thermal system I was looking at does use a stainless 300lt tank and Apricus vacuum tubes, 12v pump and fitting.
Cheers
What everyone says here is excellent advice. Solar thermal is really very efficient and not a big project if it is open loop (water array to tank).
If you live in a place with freeze issues and you go open loop, you will eventually damage the system and always have to drain it with bad weather.
I am on my 3rd open loop system in 21 years. I have always had other things to do to build a closed loop drainback, but have for others.
The Butler cost of $4,500 installed with the array can make sense if you live in a place that has good winter sun access. Anyone can do it in summer!
I am sure Butler will advise you on blending the size of the tank to the array. They are a great company and were one of the first to design for offgrid use. They were one of the first to make this easier for do it your-self or handyman type installs. The solar wand they built in California was a great solution for close-loop systems in snow country. Last, both the electric and their solar wand closed loop solved one of the biggest problems in solar hot water, system overheating and cooking antifreeze.
I have been asking them to sell the electric wand only and make the system controller adaptable for existing arrays. I do not think they will but?"we go where power lines don't" Sierra Nevada mountain area
htps://offgridsolar1.com/
E-mail offgridsolar@sti.net -
Re: PV solar panels for water heating
Hi, thanks for all the advise and various ideas. As I mentioned in my first question, I'm defiantly going to use PV panels and just want any ideas on controller options. I'm waiting for replies from some controller makers as well. I had a good look at this controller but am a bit dubious. http://techluck.com/ I know they are not a efficient as thermal panels area wise but I can see distinct advantages for our situation. My best solution so far without a dedicated controller is to charge a single battery with a Tristar MPPT controller and dump the majority output to the element. Just waiting for Morningstar to comment as I'm not sure of the efficiency of the controller when dumping and if its performance is the same as when it's charging batteries.
Cheers, -
Re: PV solar panels for water heating
Your typical MPPT controller is >95% efficient when operating over 50% of rated output (as the output drops, sun sets, battery bank is full), its efficiency does fall--But that is usually not a big issue.
Controller Efficiency does not matter (within a couple of percentage points). Batteries are around 80-90% efficient (as most people run them) when cycling... So, if you heat during the day when the batteries are full, you will save ~10-20% of your power budget by not cycling the batteries.
If you go with an AC inverter + heating element, then you are looking at another 85% efficiency (~15% losses) do to inverter losses.
Just the solar panels + controller typically add up to ~77% efficiency (mostly due to solar panel's "marketing numbers" being much higher values than most people will see due to Vmp suppression as the panels heat up due to sunlight).
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: PV solar panels for water heatingHi, thanks for all the advise and various ideas. As I mentioned in my first question, I'm defiantly going to use PV panels and just want any ideas on controller options. I'm waiting for replies from some controller makers as well. I had a good look at this controller but am a bit dubious. http://techluck.com/ I know they are not a efficient as thermal panels area wise but I can see distinct advantages for our situation. My best solution so far without a dedicated controller is to charge a single battery with a Tristar MPPT controller and dump the majority output to the element. Just waiting for Morningstar to comment as I'm not sure of the efficiency of the controller when dumping and if its performance is the same as when it's charging batteries.
Cheers,
I wonder what are you going to do when it's cloudy and the hot water tank is not hot anymore? -
Re: PV solar panels for water heating
I'll light the fire, bake some bread and heat the water. At the moment it takes 4 hours to heat the tank from cold.
I have just received from Morningstar a diagram for connecting a Tristar MPPT (for PV control) and Tristar PWM in diversion mode connected to the element. These controllers aren't that expensive. I'll try charging a battery which will also provide supply to run the controllers and since it will always be charged due to the panels being oversized for a single battery the majority of power will be diverted by the PWM controller to the element.
Just a bit more thinking about this and then I'll start spending some money.
I'll report on progress.
Cheers, -
Re: PV solar panels for water heatingI'll light the fire, bake some bread and heat the water. At the moment it takes 4 hours to heat the tank from cold.
I have just received from Morningstar a diagram for connecting a Tristar MPPT (for PV control) and Tristar PWM in diversion mode connected to the element. These controllers aren't that expensive. I'll try charging a battery which will also provide supply to run the controllers and since it will always be charged due to the panels being oversized for a single battery the majority of power will be diverted by the PWM controller to the element.
Just a bit more thinking about this and then I'll start spending some money.
I'll report on progress.
Cheers,
If you already have the system, I would simply connect the extra panels to the main system. Then I would connect the heater to the main system when needed/wanted (either manually or automatically). Much easier to do, doesn't require extra batteries, and gives you much more flexibility. -
Re: PV solar panels for water heating
We recently discussed this very thing - how to hook up PV for water heating w/o using batteries as a buffer - here: http://forum.solar-electric.com/showthread.php?21096-What-about-this-for-water-heating4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is -
Re: PV solar panels for water heating
Thanks, that's interesting reading. I'm hoping the MPPT controller will help with efficiency of the system. I have added the picture from Morningstar.
Cheers, -
Re: PV solar panels for water heating
I've been very pleased with using an AC element with my PV array for over a year now. This is with the Midnite Classic in "waste not hi" mode skimming the surplus from a 5.5 kw array. In my application, two full-sun days will trip the thermostat on the tank off (it's set at the max: 170 degrees), unless our household electricity use is very heavy.
So it certainly works even if it's less efficient than solar thermal. Solar thermal simply wasn't an option for me given the location of the sunny area of my property relative to my house. I can't comment on the effectiveness of the Morningstar design there, but the basic principle definitely works and is worth pursuing if it's the best option for you too. -
Re: PV solar panels for water heatingI've been very pleased with using an AC element with my PV array for over a year now. This is with the Midnite Classic in "waste not hi" mode skimming the surplus from a 5.5 kw array. In my application, two full-sun days will trip the thermostat on the tank off (it's set at the max: 170 degrees), unless our household electricity use is very heavy.
So it certainly works even if it's less efficient than solar thermal. Solar thermal simply wasn't an option for me given the location of the sunny area of my property relative to my house. I can't comment on the effectiveness of the Morningstar design there, but the basic principle definitely works and is worth pursuing if it's the best option for you too.
Hi, thanks very much for your positive comments. I feel in the long term, this will be the way of the future.
Cheers,
Graham -
Re: PV solar panels for water heating
Hi,
Evacuated tubes with the circulating pump powered by a small solar panel may be a consideration to heat water.5kVA Victron Multiplus II, 5.2kW array, 14kWh DIYLifepo4 bank, all grid-tied. -
Re: PV solar panels for water heating
If you want to heat water with PV, what about this:
http://www.ev-power.eu/Solar-Panels/Storage-Water-Heater-AC-DC-GridFree-Direct-Solar-100-l.html?cur=1 -
Re: PV solar panels for water heating
One 200l geyser is already on EV tubes, but for a 100l and a 50l geyser, I think this may do the job for me:
http://turboelement.com/wp/ac-dc-system-powered-by-pv-panels/5kVA Victron Multiplus II, 5.2kW array, 14kWh DIYLifepo4 bank, all grid-tied.
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