48 volt water heating element

In our community experimental greenhouse we have solar, powering pumps and fans during the spring and summer. Solar usage is an unused commodity, after fall. During the winter minimal fan usage for frost prevention. We usually have loads of sun during the winter. I have been toying with the idea of a small super insulated water heater with 48 volt elements powered by our solar. Pex running under the grow bed with a small circulating pump. Running the heater elements to keep the water at say 40 degrees, could be a big difference. The beds aren't insulated but could be easily retrofitted. Our experimental self watering beds could even have the Pex placed in and heating up the reservoir water as a solar battery, that might be better than just heating the growing medium as a solar battery. Just pondering the viability.

Is it practical to use the electricity to run a heat pump for that water?

We aren't running an inverter in our system so limited to 12-48 volts. Cost is also a factor, 48 volt heating elements are in the 25-30 dollar price point. A 12-48 volt heat pump closer to 1000.00. The quantity of water being heated could be relatively low if we use a hydronic bed system and localized to a specific bed. Part of our experiment use IBC totes to monitor water usage, will a 48 volt element be able to heat up a volume of water that large above freezing if we decide to try that?

Interesting idea! If you have unused PV solar capacity, why not put it to work?

I do wonder about the complexity though. Would simple barrels of water with heaters do the job in a more passive manner? You already mentioned that circulating fans are used in winter.

A photo or two of the greehouse would be helpful.

One of the reasons I mentioned the IBC totes is that they are already in place for a possible passive role. Creating an entry point through the lid and the element mid depth should prevent any melting issues.
Using a small water heater and being able to insulate it and  an insulated raised bed as the battery was my initial thought.

Ah! I see now -- we're on the same page then.

Aside: Another example of some posts not showing up in a thread, so I'm making comments out of turn. Weird.

I figure a solar resistance heater would be 20% efficient (assuming the solar panel is outside the greenhouse).  You could use cheap Thermoelectric Peltier-devices as a heat pump to get double the heat, (40% efficient) but even then I think you might be disappointed at there not being much heat. You could double the heat again (~80% efficient) by putting the solar panel inside the greenhouse in the winter, or using a solar thermal collector, or a reflector (like a solar oven).  Still, with passive solar heated GH, insulation (especially at night) and thermal mass are significant factors.

You could also put low poly tunnels over the beds and/or row-cover.  You can also grow things that tolerate freezing like arugula and spinach.

Check out the design features of the Chinese greenhouses; they’re one of the ultimate low-cost passive winter greenhouses. Oriented east-west lengthwise, thermal mass + insulated north wall. And especially the insulating quilt/blanket that rolls down at night.

Mike Philips wrote:I figure a solar resistance heater would be 20% efficient (assuming the solar panel is outside the greenhouse).  

Mike, I'm curious. How did you arrive at that number?

Solar to electric is about 20%, and resistance heating is about 100%

Mike, when I say solar resistance heater I see a panel with fluid that is heated by the sun, and that fluid is then used to heat or transfer heat to a solar battery. If that is your definition as well I'm quite familiar with that type of system and have used it to heat beds before. I would agree that there is a tremendous gain in using that type of system inside v outside. I'm not sure I would agree that photovoltaic panels would see a decrease of that magnitude outside v inside.
Our battery bank is actually below grade so some protection from from cold. I never considered it but perhaps a thermal recorder within the battery storage bank might be or have data that could be helpful.
We are using two styles of swimming pool passive thermo-siphoning heaters ( what I call an example of Solar Resistance Heater) to heat IBC's currently.
My thought is to use a 48 volt DC 1000w heating element in a small circulating closed loop system that with a thermal switch maintains app 40 degrees.  I don't see the entire volume of fluid being over 20 gallons. Insulating the bed and use of agri-cloth or a second greenhouse skin to help capture / maintain the heat.
Our current beds are constructed to allow for agri-cloth, shade cloth, or a second greenhouse skin to be used depending on season. One of our issues is that our location is heavily populated by chipmunks so creating beds that prevents the cute but voracious buggers out of the beds also created the framework for secondary coverings.
With our snow loads unfettered access to the first skin for removing snow prevents a second interior skin as illustrated in your drawing.

It’s not clear to me what you have down as electrical energy and what is non-electrical thermal energy.

Mike, I have two identical sized self watering beds. I have a solar array and battery bank that is under utilized in the winter.
My experiment would be to use a hydronic circulating heater utilizing a 48 volt heating element powered by the solar array to minimally heat one of the beds to 40 degrees and compare it to the non heated bed.  So the only electrical input in this experiment would be in the one bed. We would not change the way we already skin a second layer over the two test beds.
There are other beds and bed types, irrigation styles as well as heating trials that are independent of this experiment. There are beds that utilize non electrical means to scavenge thermal energy. These include heaters that I call solar resistance heaters (panels, boxes, tubes, hose) that thermo-siphon. All of the beds are constructed to be easily skinned with a secondary skin or insulation blanket for non electric heat retention through the winter.

I think the OP is pretty clear that the goal is to utilize electrical energy from PV solar panels that would otherwise be unused.

The conversion efficiency of the PV solar panel doesn't really matter, since it's baked into the system whether the output is utilized or not.

Given the high efficiency rate of a resistive "dump load" heating elementit seems a sensible way to make use of the surplus power that's available.

Will a small volume of fluid heated this way, with a circulating pump, be viable for heating grow beds? It depends on the wattage available in winter, the actual volume, and flow rate. Viable? Potentially, but there are a lot of variables in play.

Hearing 40 degrees has got me all jumpy about legionella.

Do you mean 40 degrees f or 40 degrees C?

James,
Since it is a closed loop system I'm not sure if Legionella would be an issue. The heating loop fluid could even be RV anti- freeze with ethanol and might be a good idea in case of equipment failure during a freeze. An ethanol based food safe fluid would probably be a relatively legionella resistant environment?   Hydrogen Peroxide freezes at -29 f roughly if one thinks that Legionella would be a real concern. I don't see it as an issue but open to your input.

For heating water you're either going to need an open system or expansion tanks.

I'd much rather use an open system with a thermostat at 20/22 degrees C than a closed system with any of thoses option with a closed system due to the risk of contamination with leaks or off gassing.

James,
The reservoir will be a small residential water heater. I will be running a redundant second thermostat. The pop off valve will also be in place. The minimal increase in temperature of maintaining  40 degrees won't be creating an over pressure issue. Water from my 125 foot household well is about 38 degrees there will be more gravitational pressure from weight at the bottom of the tank than maintaing 40 degrees will create in a closed vessel.  The addition of an expansion tank would be an additional redundant safety component.  Water expansion from 0c to 3.98 c is listed as having no real thermal expansion. There is a possibility that both thermostats could fail and the relief valve could fail, so maybe an expansion tank should be added. If the fluid is an alcohol based coolant an open system might not be an good option due to evaporation. If I were to use my own H2O mixture and use methanol, should there be a leak it wouldn't be dtetrimental to the plants it seems where a food safe GRAS ethanol based antifreeze might inhibit some growth. Dilution of the alcohol in 20 gallons of coolant v 150+ gallons of H2o in the contained self watering bed would be significant.

I would be concerned about an alcohol component in a heating system. In a closed system, its evaporation will create pressure or cavitation in the pump. And as you note, in an open system it will evaporate off.

Propylene glycol is widely used in cooling systems in food facilities as it is non toxic. I wonder about a low temperature heating system -- might work?

A low tech option would be salt water, which holds more heat due to its greater density, and is less friendly to bacterial growth. However, it is corrosive to metal parts, including the heating element.

This is a low temp set up so my assumption is that there would be little alcohol evaporation in the constantly moving heating fluid. With our winter sun the greenhouse warms up nicely, I am just trying to keep the night time temp at that slightly warmer 40 degree temp.
RV antifreeze with a pure alcohol component has about 35% alcohol.
The propylene glycol and alcohol mix varies, antifreeze with just propylene glycol as the component has 25-30 % water. Propylene  glycol does inhibit bacterial growth but again in the closed system I don't think legionella will ever be a concern.
Salt  would cause an issue if there was a system failure.
I'm thinking that RV antifreeze will be what I end up using

Your well is at 38 degrees C??? If you're working in fahrenheit, then 40 degrees isn't a worry for legionella and everything I've said is moot.

So please, specify the units you're using.

For people to learn from what we write here, we need to be clear in what we're saying.  

James,
I guess we both took it for granted in our units of measure. Posting from the US I used my everyday common units of measure and you used yours. When in Rome do as the Romans do, comes to mind.  Situational awareness is a good thing and I am in Oregon. I can easily find a yardstick but have yet to see a meter stick at the local hardware store. 60 kmh is a bit slow for a US hwy but the darn signs here just say speed limit 60. You are absolutely right for a global audience f and c is important.
I wish that the metric system would become the US standard. I see it often when using tenths,hundredths thousandths of an inch in a discussions, my customers just nod their head. Mils are so much easier to use.
I wish my household well was @ 40c I would heat with it and just pump that through the greenhouse.
In a closed loop system @ 40c it would be a good temperature for legionella to thrive but inhalation exposure would be mitigated by the roughly  450 mils of soil and 450 mils of gravel+ water it would pass through should there be a pex failure.  Composition of the fluid is an important aside, both polyethylene glycol and ethanol are bacteriacides.

Robert, it would be interesting to know how much surplus PV electricity you have available in winter. It takes a substantial energy input to make the rest of the system worth the effort.

My battery bank consists of 8 Trojan T-105's they are rated 225ah @ 20hrs. 8 Zamp 12v 150 watt panels. There would be no other components running over the winter.

Cool. That's serious juice man. I'm envious.

Have you looked into heating elements? I know you can get 12v, I suspect you can get 24v, and I can't think of a reason why you can't put them in series for 48v.

Douglas Alpenstock wrote:Cool. That's serious juice man. I'm envious.

Have you looked into heating elements? I know you can get 12v, I suspect you can get 24v, and I can't think of a reason why you can't put them in series for 48v.

I think this would be the best route. Cheap and easy to configure.