Solar Thermal is Dead?

semipro Posted By semipro, Apr 24, 2012 at 11:35 AM

  1. GaryGary

    Feeling the Heat 2.

    Jul 12, 2010
    SW Montana
    This was what Martin put forward in his blog that stared this thread -- that maintenance on a combined PV + heat pump water heater would be less than a solar thermal system.

    I find that a bit mind boggling.
    On a PV system you are likely to have to replace the several thousand dollar inverter at least once and perhaps twice in a 30 year life. And, if you have to add the heat pump water heater to get the efficiency up to a reasonable level, you are likely to have to replace the heat pump water heater ($3000?) at least once and probably twice during the 30 year life. You might well be talking $12,000 of maintenance over the life of the system -- that does not seem small to me?

    I have two drain back water heating systems (space and domestic water) -- the oldest one being about 9 seasons old. The only maintenance has been an early failure of the Goldline controller which cost me all of $100 to replace. I think drain back solar thermal systems are about as maintenance free as it gets.

    Maybe I'm missing something?

    Agree that if you have access to NG its getting hard to beat with anything but DIY solar thermal -- except from a carbon emissions point of view.

  2. webbie

    Seasoned Moderator 2.

    Nov 17, 2005
    Western Mass.
    I had to replace my $1000 stone lined solar tank in less than 10 years......

    I guess the real answer, as usual, is "it depends".

    A simple solar thermal tank heater, like they use in much of the world, can't be beat. But the more you have to add in plumbing, controls, asme storage, anti-freeze and other stuff - the worse it can looks.

    I'm thinking that in the future they will be able to avoid the inverter??? Couldn't they just send current direct from the panels to a heating element of sorts?

    Even if not, what will inverters cost in 10 or 20 years?

    I'd say this is all a bit premature - that is, PV seems cheap due to the glut and other things. But I can imagine that someday....the idea of a single wire from remote mounted panels...instead of plumbing...will be tempting. I doubt I will see the day though!
  3. woodgeek

    Minister of Fire 2.

    Jan 27, 2008
    SE PA
    I share Gary's horror (despite being a nay-sayer on professionally installed solar thermal in 2012) at using PV to drive a resistive element.

    In Semipro's case, he already has a Geospring, which we can assume gives a COP ~ 2 (if he is a low DHW volume user). Thus in round numbers every solar kWh you put in only reduces your grid usage by 1/2 kWh! Moreover, when that unit is heating a cold tank, the COP starts high (>3) when the water is cold due to lower 'lift', and then drops to <2 at the final, high lift part of the cycle. If you are only preheating to a 'tepid' level, then you are only saving the COP=3 part of the cycle, and saving 1/3 kWh from the grid per PV kWh! Yuck.

    IOW, the Geospring (that is already installed) is so efficient that it nearly negates any savings you might get from solar preheating.

    In PA, HPWH and professionally installed solar thermal can both cost similar amounts installed after rebates, (In my case quotes at ~$3k and $4.5k, respectively), and either will def use less than half of the kWh of a conventional electric, if installed properly. Both systems are 'complex', with low unit sales hardware which leads to maintenance problem risk. Simple payback on either system at $0.15/kWh is 5-7 yrs relative to a conventional elec tank, depending on DHW usage. Which you pick depends on your solar site/resource and if you have a location for the HPWH.

    Installing BOTH systems (each capable of saving you >50%) does not get you to zero grid kWh by a longshot, both are awesome in the summer (lots of sun and warm air) and suck in the winter (both can go to elec backup). You could still need 25% backup, and the payback could be >8-10 years.

    If you want to 'go solar' and have the site for it, do conventional grid-tie PV, cash your rebate check and let the Geospring do its job.
  4. jimbom

    Combustion Analyzer 2.

    Dec 19, 2010
    Missouri Ozarks
    I haven't completely achieved a functioning state yet this morning. My initial thought is a kWh from solar is equivalent to a kWh from the grid. Regardless of the application that it powers.
  5. btuser

    Minister of Fire 2.

    Jan 15, 2009
    Principality of Pontinha
    If it's ground-level passive without the need for freeze protection I don't think you can beat solar thermal. I'm planning on working it into a greenhouse project (should get to that project by oh, say 2021) to extend the season.

    Is there any danger to harming PV panels when you're not using the current ? Do they get "backed up" if you're not dumping into the grid or somewhere?
  6. semipro

    Minister of Fire 2.

    Jan 12, 2009
    SW Virginia
    Great insight. Point taken. I hadn't thought of it that way.
    However, our situation is more complex than I made it sound.
    The GeoSpring HPWH in the basement is one of two water heaters. Each serves different zones. The other WH, an electrical resistance unit is located where its hard to service and heat loss to the outside is a problem. Also, if it were to leak it would be really bad.
    I want to get rid of the 2nd WH and combine the zones for a variety of reasons.
    So basically I need to increase the capacity of my system to compensate for the loss of the 2nd WH. I was thinking that 40 gal (or more) of water preheated to 90 degrees prior to the GeoSpring HPWH would take care of that.
    Also, my HPWH is basically "stealing" heat generated by my ground-source HP HVAC in the cold season so my HPWH COP is really lower then.
    Also, we have hard water and don't want to soften it. The only issue we've had with this is that electrical resistance WHs soften the water by removing the minerals in a way that is harmful to the WH and requires frequent maintenance. This is one reason why we invested in the GeoSpring HPWH and why we want to get rid of the 2nd electrical resistance WH.
    And, the presence of a preheat tank or tanks in my basement allows me to recover energy from other sources if I want. I'm thinking for example of waste water heat capture.
    I also really want a small wind generator even if its not economically practical. I'd considered taking the power it generates and preheating water with it, again avoiding the complexities of battery storage, or inverters.

    BTW the system I made calculations on in the last post has a payoff period of 6 years assuming no financial incentives like tax rebates. It would also seem to have "zero" maintenance over 25-30 years unless the preheat tank or heating element go bad.
  7. woodgeek

    Minister of Fire 2.

    Jan 27, 2008
    SE PA
    Ok. So the systems to cost/compare are Solar Thermal, Solar PV straight to a conventional tank, or adding a second ASHP (or upsizing to something bigger like the AOSmith 80 gal ASHP and selling the GE). Cost will need to factor DIY-fraction, rebates, and install details like solar resource (trees) and ASHP siting (scavenging from house or not). I would still think that your best options (on a cost of ownership over >8 years basis) would be DIY or DIY-kit Solar Thermal OR ASHP, perhaps in a site different from your current one.

    Oh yeah, my recent reading has suggested that the scavenging issue is not such a big deal as we might think intuitively. IF the BTU is coming from a properly installed geo, I would say the effect may be almost negligible.

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