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Solar-powered resistance hot water heater

Post in 'The Green Room' started by begreen, Oct 27, 2012.

  1. begreen

    begreen Mooderator Staff Member

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    A while back there was some talk about this method of heating water with solar power instead of a hydronic system. Essentially it uses solar panels to drive a 1000 or 1500watt element in the hot water heater. Can someone provide me a link to systems set up like this or the earlier article. I am toying with the idea but I need to see how practical it would be.

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  2. raybonz

    raybonz Minister of Fire

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    Info here BG.. http://sel.me.wisc.edu/trnsys/downloads/trnsedapps/demos/pvsdhw.htm

    Ray
  3. begreen

    begreen Mooderator Staff Member

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    Thanks. Are there products on the market for these systems or is it so far mostly theoretical?
  4. raybonz

    raybonz Minister of Fire

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    At the bottom of the link I sent there is contact info that may be your best option. Sounds like a good idea to me and much easier to install than a piped system..

    Ray
  5. GaryGary

    GaryGary Feeling the Heat

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    There was also a writeup on the http://www.greenbuildingadvisor.com/ site about using a PV array and a heat pump water heater. Since the Heat Pump water heater has a COP (efficiency) of about 2, this raises the effective efficiency of the PV panels to 25% or so -- still pretty dismal.

    To me, the big drawback of this idea is the large area of PV panels you would need. If a 60 sf solar thermal collector system would be about right for your family, then you would need about 240 sf of PV array to get about the same performance. If you add the heat pump water heater to get up to a higher system efficiency, you end up with more cost, more complexity, and still need more panel area than a solar thermal system.

    I suppose I'm an incurable solar thermal fan, but the article saying that rock solid HVAC circulator pumps that are commonly used in solar thermal systems are unreliable when they typically last 20+ years with no service seems over the top to me.

    A simple drain back solar thermal system has only one moving part (the pump), and a simple differential controller that is certainly no more complex than the microprocessor controller used in the simple PV system at the link listed above.
    http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm

    Gary
  6. Dune

    Dune Minister of Fire

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    Begreen, why? Hydronic is far more efficient.
  7. begreen

    begreen Mooderator Staff Member

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    Just weighing the pros and cons. Hydronic may be more efficient, but electric is much simpler. It's easier to install, less unsightly, has no drainback requirements or freeze protection issues, minimal plumbing requirements, etc..
  8. Dune

    Dune Minister of Fire

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    Still way more expensive in spite of that.
  9. peakbagger

    peakbagger Minister of Fire

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    There is a lot of discussion currently about using a heat pump water heater with grid tie solar. With panels around $1 a watt and the ability to "bank" power to when you need it, its real hard to justify putting in a regular SHW system especially since SHW systems work the best in summer when most dont want the heat and work poorly in winter when the ground water is cold and you want the heat. I have a regular SHW system and if I was doing it again I definitely would have extra PV panels instead.
    woodgeek and raybonz like this.
  10. Dune

    Dune Minister of Fire

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    OK, I do come here to learn afterall.
    raybonz likes this.
  11. velvetfoot

    velvetfoot Minister of Fire

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    The PV grid banking concept is huge. No need to futz around with special equipment, right? Just use an electric water heater.
  12. woodgeek

    woodgeek Minister of Fire

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    As peakbagger said, use a heat-pump based unit. A properly installed HPWH will use about the same total kWh per year as a 'typically installed' solar hydronic DHW system will using resistance backup. free solar BTUs are never free.
  13. peakbagger

    peakbagger Minister of Fire

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    Tom in Maine who posts on this site sells a Geyser heat pump water heater that will use a regular hot water heater for its storage tank. If you need a new heater maybe get an integrated one like the GE unit but otherwise why junk a tank unless your water is real agressive? I think the elements stay there so if the heat pump water heater isnt keeping up, the resistance heaters still work.
  14. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    If you take a family of 2 with a demand of 50 gallons of hot water a day total, and heat it from 50F to 120F, thats (50 gal)(8.3 lb/gal)(120F-50F) (1 BTU/lb-F) = 29K BTU per day, or 8.5 KWH per day. That's 3100 KWH per year. Don't know where you are, but if I run PVwatts for Springfiled, IL (a nice central place), it takes a 2.5 KW PV array to generate 3200 KWH per year -- just enough to fully satisfy hot water energy on a yearly basis.

    Just as an example, one 230 watt (Hyundai) panels is 65 by 39 inches or 18 sqft, or 13 watts/sqft. So, 2500 watts would be almost 200 sqft of PV panel to satisfy hot water demand. This compares to a typical two 4 by 8 solar thermal collectors (64 sqft) that would satisfy about 80% of hot water demand. This is just fact that solar thermal collectors are about 4 times more efficient than PV panels (50% vs 13% more or less) -- this 4 to 1 is a big factor, and IMHO more than overcomes the disadvantages of the more seasonal output of thermal collectors.

    Not sure what this 2500 watts array would cost today? Solar Today did a nation wide US study last summer and got an average of $6.80 a peak watt -- its probably down some from that, but at that price, it would be $17000 (installed and before rebates). In Mt (where I am) that would qualify for 30% federal + $1000 Mt credits for a total of $11000 out of pocket. Again, that is probably on the high side as PV panel has be dropping.

    Our local solar water heating installer (Liquid Solar) does 2 panel thermal systems for about $5000. In MT they qualify for the 30% federal credit as well as an MT $1000 per couple credit, so end cost comes out $2500 out of pocket. I think we are a bit lucky to have this guy who does great work at somewhat lower than average prices, but shopping around might find a similar guy where you are.

    If you went with the heat pump water heater to improve the overall efficiency of making hot water, it would cut the required PV array for water heating about in half, but add a system that is inherently more complex that a whole solar thermal water heating system. Not sure what they cost, but it does seem like it might work out since it would lower the cost and reduce the roof space taken up by PV panels. But, it seems like a much more complicated (overall) system than a solar thermal system.

    Tom is mentioned above as a supplier of heat pump water heaters -- he also now offers a $2800 solar thermal hot water system -- http://www.americansolartechnics.com/4.html that is easy to install.

    It seems to me that there are only two reasons that a PV system for hot water is even in the running at all are:
    - The solar thermal systems are a lot more expensive than they should be. The solar thermal industry just seems stuck in the three decade old mud, and has just not improved the competitiveness of the system over the years. One indication of this is that you can do a good DIY system for a bit over $1000 while commercial systems are up towards $8000. There are some signs of change here, like Tom's system, and the Fafco system -- hopefully more coming. This is one I'm working on: http://www.builditsolar.com/Experimental/OffShelfDHW/OffShelfSolarDHWV2.htm

    - The netmetering system that PV systems benefit from. This basically "makes" your utility pay you full retail price for any electricity you generate that is in excess of your current needs. It allows you to bank energy in the summer against shortfalls in the winter. This is an great deal for PV owners and a poor deal for utilities as they could buy the same power delivered exactly when they need it for about half that price. It basically amounts to a subsidy to customers with PV that is paid by the other utility customers who don't have PV. Its an encouragement for people to put PV systems in, and I think that's good. But, utilities are only obligated to do this until PV grows to a certain percentage of their demand -- often 2%. Some areas are bumping this limit and in these areas utilities are wanting to renegotiate the deal.


    Just as an aside, you can size solar thermal systems to provide near 100% hot water for the full year by somewhat oversizing the collector, going to a higher tilt angle that is more optimum for winter, and less likely to overheat in the summer, and using a larger storage tank. In my current system, the collectors are vertical and oversize and they do quite well in the winter -- when I checked earlier today, the solar tank was at 161F. I doubt if I have more than 4 or 5 days a year when my tank temperature drops below 110F.

    You mentioned water supply temperature variation over the year. I was not able to find much on that in a quick search. Ours is from a well and hardly varies at all. How much does your's vary?

    Gary




















    The temperature of groundwater fluctuates considerably less than air
    temperature because groundwater is insulated from surface-temperature
    extremes. Groundwater temperatures remain fairly constant throughout
    the year. This property makes groundwater useful for heating and cooling
    systems. Groundwater is warmer than the air in the winter and cooler
    than the air in the summer
  15. peakbagger

    peakbagger Minister of Fire

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    As for adequate winter hot water I guess it depends on what type of SHW panel. My flat plate AET panels are way down on the efficiency curve at 80 degree differential between the outside temp and the panel outlet temp. If I went evacutated tubes I could go much higher albeit at higher initial installed cost. Tom's heater is effectively a flat plate so its down in the 80 degree F differential before it runs off the curve. In Northern NH I do get a four months of 30 degree weather or below. If I assume zero differential between the glycol temp and the hot water, that means 100 deg F hot water which is border line for my use as there is some stratification in the tank. This is with my collectors mounted at latitude minus 15 degree so I have them set to optimize winter production. I expect if I got close to stagnating the system I might be able to get a bit more temp but for most they would need to supplement. During the summer the system bascially sits there generating way to much heat that I have to manage.

    I agree net metering is a no win situation for my utilityand other ratepayers but I take advantage of it like folks take advantage of federal rebates for installation of systems. In state with Feed in Tariffs, the subsidy is even higher as normally the consumer gets paid a high renewable rate for all production, then gets to buy it back at the regular rate and even possibly off peak. This can effectively reduce their rate to 1/2 or 1/3 of the standard power cost. I agree that the economics of conventional SHW is steep and this drives the discussion along with utility incentives.

    As for the installed cost of PV, the cost for incremental PV is not the same as for the initial installation. If someone pays to install a new PV system there are fixed costs and variable labor plus incremental costs per watt. The cost to design and permit are fixed along with for the sake of argument the installers expected profit for each job. The crew is paid for travel to the site which is relatively fixed and realistically they are paid by the day as few installers would work on two installs in one day unless they are in a urban area. As long as the incremental PV does not require them to hang around an extra day, the labor per watt goes way down as they are already on site and mobilized and budgeted to be at the site all day. Then it comes down to the incremental cost for panels at a wholesale of $1 per watt possibly additional inverter size which again with a central inverter is less $ per watt for incremental or at best $1 per watt for microinverters and for sake or simplicity $.50 per watt for racking and hardware. So the incremental installed cost is probably closer to $3 per watt. By banking the power in summer and going with tankless electric I expect the ecomomics will change especially if someone has access to a FIT.

    In order to come up with the best solution its going to require taking actual use and local conditions into effect. I expect someone with a time to build a spreadsheet could figure it out but as it is site and use specific I will let someone else do the math. In some cases thisSHW will make sense and in other it will be PV.

    I personally use SHW for 6 to 7 months per year exclusively and then use it for preheat in the winter with my wood boiler doing the rest of the work and oil as a final backup..
  16. begreen

    begreen Mooderator Staff Member

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    Good info folks. We already have 3K grid-tied solar and may just add on the system.
  17. DBoon

    DBoon Minister of Fire

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    That, too me, seems about double the hot water that two adults reasonably need in a given day. There are many people on this site who have weighed in on this over the years. Our two person usage averages out to ~30 gallons per day (about 85 kWh/month with our HPWH). Granted, we have all appliances with very low hot water usage, and we don't take long showers. Still, others do even better - sometimes half that.

    So, if you consider that a reasonable two person usage is somewhere in the order of 1.5 to 3 kWh/day, and the prices I have been given for installed solar thermal in the range of $8-10k (and not even covering 100% usage), it is easy to see why many people would find PV electric a better option, especially when we get the cooling and dehumidification for free as a by-product of the HPWH.

    I can see a large family or commercial business benefiting from solar thermal, but not a two person household.
  18. pdf27

    pdf27 Member

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    Due to our rather screwed up subsidy system in the UK quite a lot of people are starting to use devices like http://www.immersun.co.uk/ to heat water. Essentially we don't have net metering and the amount of power you export is assumed to be 50% for small installations rather than measured.

    The net result of this policy is devices like Immersun which ensure that you don't export any of your PV generated electricity and instead it's sent to a dump load (normally an immersion heater in the water tank). Heat pumps won't work well with variable loads like this, so you're stuck with electric resistance heating. From an environmental point of view it's madness, but unfortunately it makes sense from a subsidy point of view.
  19. woodgeek

    woodgeek Minister of Fire

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    I suppose it is no 'worse' than dumping extra solar hydronic heat in the summer, for comparison. Or battery in-out losses in an off-grid system. Without a grid battery, some choices need to be made.
  20. nate379

    nate379 Guest

    My 4k grid tied system cost me about 1.75/watt all said and done. Canadian solar panels, enphase inverters. Did the install myself. Wasn't rocket surgery by any means!

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