I have an interesting project coming up - I'm planning some public restrooms/showers to serve a public dock area in Rhode Island. The restrooms will be open about Memorial Day through Columbus Day. There will be two (maybe three) showers for each gender and I have no idea how much use they will get although it will likely be heavy. I'd like to heat/preheat the water as much as practical with solar. The building will be new but has to fit a footprint that places it's long axis within 15 degrees of east/west - so that works. I could just plaster the roof with panels but I'm concerned I'd need a massive heat dump and I don't know how I'd do that. There will be a lot of excavation in the area around the building - should I be dropping loops of pex in the excavated area for a future heat dump. Also, I anticipate this system will get drained during the freezing months. Any ideas would be appreciated.

The simplest? I would setup a batch heater (big black tank) on the south facing side that in turn fed the regular water heater. The more sun you have the warmer the pre tank would get and wouldn't overheat. The only drawback I can think of would the tank would cool at night, although it might still be above the incoming cold water temp.

Interesting idea..... Two things come to mind, though. Since the hot water demand will be for showers I suspect the peak demand will be in the early morning - that might be after the batch tank has lost a lot of heat overnight. Also, aesthetics are a concern here, since this little building will be part of a rather iconic "New England wharf" sort of setting.

I also have to work within a limited footprint so space is a bit of a premium. I was wondering, can tankless heaters operate so as to account for varying incoming temperatures? In other words, if the tankless output is to be, say, 125F, does the tank "know" when the incoming water has been preheated to say 110F and thus throttle back on the gas? Or do they just sense flow and fire at a constant rate until the flow stops?

Good point and good point. I was thinking more like a solid black wall, similar to wall air heaters. But the morning issue would put a big dent in that and still look a little odd.

I don't know about gas units but the electric ones I had would just shut off, which was annoying because at some point they crossed the magic line. So as you took a shower and the pre tank didn't need heating you had to turn the water all the way hot, then it crossed the input temperature threshold and kicked on and suddenly I had a really hot shower. I think using a small gas tank (40 gallons?) would be a better way to go.

First things first - don't let all that hot water go down the drain without a waste water heat recovery device.

Since the load will be variable and to some degree unknown then I'd guess that you would want some rapid response "backup" system like a gas tankless. I'm pretty sure they can deal with a variety of incoming temps to deliver the desired output temp provided they are not undersized.

Since this is at a dock location and will be used primarily in spring through fall it practically screams to me to use a water to water geo heat pump using lake loops. During the summer the unit can also A/C the building while dumping the heat into the shower water storage tank. Of course also heat water with solar panels at the same time.

It might make sense to heat the water sequentially as follows:
1) Incoming water heated by waste water recovery exchanger(this should work very well for a shower facility)
2) Then to a first tank heated by geo up to 100 or so,
3) On to a second heated by solar to 120 or so.
4) On to the point of use through a tankless gas water heater

Have you determined how much energy there is to actually make use of by doing calculations on the roof, and then making some assumptions about how the water is going to be used throughout the day, and then, how much storage you'll need in order to store all of the heat?

For example, if you install a drainback system with water (no glycol) - which you might be able to do since it's a summertime system, you might have no heat dump requirement. Given that, you should be able to calculate typical BTU's in through the panels, model a shower scenario, and determine how much storage you need.

BTW, it does not strike me that the heavy time will be first thing in the morning, unless you are servicing a campground.

Jeff

If you want to dump heat when the system is hot, just put a thermally activated valve ( http://www.fpevalves.com/productsThermostaticControlValves.asp) on the solar return to the roof and then plumb some high capacity baseboard to it as a dump zone. The dump zone can be outside on the way back to the collectors or in an attic underneath.

I'd say you want roughly 1 ft of high cap baseboard for every 8 sq ft of collector area and you should be able to dump all the heat you need to. Maybe more like 1 ft per 5 sf if you are using evacuated tube collectors.

That is not to say that you shouldn't do some homework to figure out what the load is going to be because even if you can dump heat, why would you want to?

~F

Thanks for the rule of thumb - that's helpful. It's hard on this project to predict usage since it's essentially public showers. I think the demand will be high and I'd like to design to provide as much of that as possible. The input should be easy to determine but if for some odd reason no one shows up to take a shower I could have lots of very hot water on hand - hence the dump requirement. Maybe I could valve it out to the dock supply and let everyone wash their boat with hot water!!!

Greg,
This building is a slab on grade beams on wood pilings so I don't have any access under the showers. My understanding is that wastewater heat recoverers have to be installed vertically and are about 4 to 5 feet long - No can do here, but i like the idea.

A heat pump is out of the question - this project is on a small island 12 miles out in the Atlantic and has the distinction of having the highest electric rates in the US - last June they hit $.68 per KWH!! Running a compressor is not a good way to go. But ordinarily that would be a good and logical suggestion. Also the water is salt and has a lot of private and commercial fishing boats so I'd be concerned about them fouling the loops and marine growth on the loops.

Jeff,
I'm still looking at this conceptually at this point so I haven't ground any numbers yet. Is there a good source for typical panel outputs in a given location?
This will be in a sort of marina type location so there will be both early risers and later ones (they'll be hung over so will probably take longer showers!). There may be usage all day (certainly the HW side of the lavs) but I suspect the peak will be say 8:00 AM to 11:00 then again in the early evening (5 to 7 or so) as folks return from the beach and prepare to go out for the evening. This is a total guess - I have no data or observations to go by. My concern is that 2:00 PM July sun could overheat the system.

Again, you will not have excess heat or hot water if you are able to turn the system off (when the tanks are full) without risking fouling of any anti-freeze. There are plenty of resources for solar hot water, including an excellent, if a bit folksy, book called "Solar Hot Water Systems: Lessons Learned 1977-Today" that was my bible in designing the 5 panel system for my house that is now being built.

The National Renewable Energy Laboratory publishes TMY weather data that you can use (see http://rredc.nrel.gov/solar/old_data/nsrdb/tmy2/) to model your system, if you can understand it. If you cannot understnad that data, and how to translate that data into a model for your custom system, then I think that you should find professional help.

The fact remains, though, that you must find find some data and make some assumptions about how the showers are going to be used. For example, if the ground water is 50 degrees and the shower water is 100 degrees, one 5 minute shower will consume (1 BTU / degree / pound) * (50 degrees) x (2.5 gallons / minute) x (5 minutes) x (8 pounds / gallon) = 5,000 BTU. Also, since most of the sunlight will come in during the 5 or so hours around noon, you will need to be able to store a large fraction of the heat that you collect, as most of the showers will be taken away from the full-sun hours.

On the tank side, one 120 gallon tank (about 1000 pounds) can store about 50,000 BTU's if you assume 50 degrees of temperature rise during the day. (You might be able to get more rise, though.) That's only 10 showers. If you are planning on 40 showers a day, then, you will need (according to these back-of-the-envelope calculations) a system that includes 500 gallons of water storage, and probably 8 or 10 4x10 panels at a good angle. Is that reasonable?

Jeff

Birdman, I worked for a couple of year delivering (moving) sailboats from one place to another around the great lakes. So confirming what your guess is as to when the showers are used and I can assure you the mornings will be busy, usually from sun up to about four hours after that and then again close to sunset.

For the drain water heat recovery they do have units that are similar to what a boat uses, basically draining in to a small basin that is then pumped up (similar to a sump pump) to then fall through the recovery unit and out to the sewer. So that might still be an option. Honestly I think it would be the biggest bang for your buck energy wise.

There are horizontal options:

http://www.ecodrain.ca/en/how-does-it-work


I'm not sure how clog/maintenance free or effective this horizontal unit really is, but even if it's only half as efficient as the vertical versions it's worthwhile. At a half-buck+ per kwh you can afford some plunger time, if that's what it takes to cut the water heating bill in half. (Half  is about what a 4" x 4-footer  in the vertical versions give you in a shower or carwash type of application.)

BTW: You get more out of 'em if it's plumbed to feed both the water heater and the cold side of the shower mixer.  Plumbed shown is the LEAST effective configuration (but typically the easiest to plumb as a retrofit).  If feeding the cold to the water heat as well you'll have 4-5x the potable-side flow. It's output will be somewhat cooler, but the total heat transferred will improve by more than a third.

And if a few of these cuts the size & cost of the solar array necessary to support the load by something like half they've more than paid-for itself up front, eh?

Haven't seen 3rd party test data on these (yet), but in your situation it may be worth sampling one or two and doing some side-by-side field testing.

An alternate heat recovery approach might be to build a large (500- 1000 gallon?) insulated greywater tank installed at the outflow of all shower drains and put a submerged heat exchanger in it (like a big coil of PEX or something), and put the heat exchanger in series with the coldwater feed to all water heating systems.  (There will be code issues to be mindful of, and the tank would have to be cleaned at least annually, but it can work.)

Interesting project Birdman. It seems important that the project specs should specify the number of gallons available at x temperature per day or perhaps even at specific times of the day. If they are expecting you to determine these requirements for the facility then they should be paying you a significant fee just to do the research on what their needs are. Even then they need to be responsible for defining to a fairly specific degree what performance they are expecting. Unless of course they are willing to accept any result.

A shower facility in a high dollar electric environment is the absolute perfect match for a drainwater heat recovery device. I'd encourage you to look hard at figuring how to work one in if possible.

Wouldn't extra tanks be the best way to dump excess heat? That way the heat still might be able to be utilized the next day when rain clouds roll in.