Solar Hot Water Heater Question

[Paddy]

Raspy,

I read your link to "bad experince with evacuated tubes. Looks like one brand 10 years ago. The technology is solid.
I keep hearing folks talk about heat per sq-ft. Unless you are filling your roof with many panels, it's more about cost of a panel and how much heat will it generate. The issue is flat panels do not work well in winter or in cold conditions. The flat panel works on the same principle as a garden hose left on your patio. Sure get hotter than snot in the summer, but frozen solid in the winter

There are million of Evacuated heat panels in China. I see very few flat panels there

 

[Industrial Toys]

Something I have wondered about. You (Raspy) say to keep the collector cool. So is an increased flow of water desireable? How does one establish this? If you move twice as much water it is only half as hot, no? In fact, a thermo syphon system needs no pump, but is darn slow so is this in fact wasting BTUs?

 

[Raspy]

Paddy,

You're right about the cost per performance. It's cost for BTU. All collectors work like your garden hose left on the patio example, even the evacuated tube. A dark surface absorbs energy and that energy is transferred to a storage media, like water.

The transfer is the interesting part. A garden hose is not insulated or glazed so it reaches equilibrium at a fairly low temp, where losses equal gain. A good collector addresses this problem, when higher temps are wanted, by reducing environmental losses so more is available to go to the storage media. This is done with insulation, selective surface absorbers and low iron glazing. And very importantly, by running the collector as cool as possible. This is done by "cooling" it with the circulated water, which gets hot in the process. It sounds backwards, but solar systems should be designed as cooling systems, where the coolant is the water you want heated.

I haven't run side by side tests to see what produces more per cost, but I've had excellent success with flat plates and if they freeze in the winter there is something fundamentally wrong with the design. Collectors, including flat plate collectors work best when the sun is out, obviously, and it has little to do with winter or summer. Even cloudy days produce useful energy. There must not be any chance of freezing and there must not be any energy used to prevent freezing, such as electrical heaters or recirculating the heated water.

As an example of good performance and reliability, my system has (6) 4x10 flat plate collectors that heated my 2,800 sq ft house last winter with no backup heat. No freeze ups either, and it's rated for 100 MPH winds. The glass is strong enough to be walked on and support snow loads. Performance cannot be lost because of lost vacuum and there is no water stored up on the roof, which means it's impossible to freeze. During the warmer months I have 650 gallons of hot water on tap for unlimited hot water and other future uses.

 

[Raspy]

Something I have wondered about. You (Raspy) say to keep the collector cool. So is an increased flow of water desireable? How does one establish this? If you move twice as much water it is only half as hot, no? In fact, a thermo syphon system needs no pump, but is darn slow so is this in fact wasting BTUs?

First let me say that double glazing, generally, won't help you. The reason is that each layer of glass filters out about 8% of the energy. Second, we are not trying to get the highest temp possible out of the collector, we want useful heat with minimal losses to the environment. A double glazed collector, sitting in the sun empty, will get hotter than a single glazed collector, because it has a higher insulting value, but it cannot produce more BTUs at normal working temps because of the extra filter.

Remember the basic design idea: We want as much energy as possible to go out of the collector in the water and as little as possible to be lost through the insulation, the glazing, or other ways. This means we should go for a small rise in water temp for each pass through the collector. It also means we want the entire absorber to be as close to the same temperature as possible, which means no areas of the plate are warmer than others, which means close tube spacing in the collector and relatively high flow rates. Never design the system to make very hot water on each pass.

Thermosyphon works by heating the water sufficiently to make a difference in volume, which means the hot will rise to the top. It means some, or a lot, of the collectors energy is used to drive the circulation so less will be delivered to storage than if it's pumped and more is lost to the environment. It also means the storage is above the collector and that is a problem in most designs. I know, pumps take energy, but with a pump you can have the storage below the collector and you can have excellent freeze protection.

Bottom line: Twice the volume at half the temp rise, beats half the water at twice the temp rise. This concept then begins to call out storage volume per collector square footage and it's a big part of how energy is delivered to the house. For example, in-slab radiant is far superior to baseboard heaters, when driven with solar. Large domestic storage tanks, running a bit cooler, are better than smaller storage tanks that are hotter, within reason, as the energy still has to be at a useful temperature.

But before you get to all that, the FIRST thing to consider, before sizing or anything, is freeze protection. And remember hot does not mean a lot of energy. Energy is described as BTUs. This is measured in how many pounds of water were raised how many degrees. Temp is only part of the measurement.

See how interesting solar design is? So many variables and so many theoretical ideas. Spend on collectors and storage, not on parts you don't really need. Never forget to seek what is the most practical. Simple foolproof designs are sometimes difficult to achieve. I've seen outrageously complicated, over designed setups that were doomed to fail and wasted a lot of cost on non productive equipment. Other designs that, in theory, had some advantage, but brought along other problems, like low durability, or higher temps with lower BTUs, or cheap freeze protection that was bound to fail, etc.

 

[Paddy]

Raspy,

different climates, different results. What is your average winter daytime temperature in N. Nevada? The op is in Canada and your system will perform very differently in Canada. The advantage of the evacuated tube is the insulation value of vacuum. It's R = 100/inch. There are nearly no losses due to cold ambient temps. If using for home heating this is the only time of year it's being used.

Heat transfer is based on delta T. The greater the difference the more heat flow. As we all know keeping your house at 62 F is exponentially lower cost than keeping it at 72 F. Delta T plays two roles, storage transfer and at the collector transfer. The cooler the storage tank and the cooler the return water is in the manifold, the better heat transfer. Ideally for radiant I use 105 F, anything below 95 F has little use. A charged storage of 180 F and using a mixing valve is ideal. Evacuated tube panels can heat to nearly boiling point. So charging an exhausted storage tank starting at say 100 F, has a huge delta T. Through the day as the tank is heated, transfer slows.

I was going to used solar water heating for my radiant but, I'm planning on PV so I can harvest year long

 

[Industrial Toys]

Thanks for all the great information. Paddy, you say transfer slows. Why? This may be a stupid question, but cold water doesn't get heated any faster then warm water the way I understand the physics. Just for simplicity, let's say with an electric element. As the water temperature approaches the temperature of the heat source, does transfer slow?

 

[Paddy]

It does heat slower if the temperatures are nearly the same. Formula;

Q=m x c x Delta T. Where m = mass, c = specific heat and Delta T is the difference in Deg Kelvin.

Image placing two rods of steel in a bath of water. Bath is 200 F, one rod is 150 F and the second rod is 70 F. Eventually all will be the same temp. The 70 deg F rod will rise dramatically vs the 150 F rod

 

[Industrial Toys]

I am having a little trouble understanding that. I once had an eight wheeled ATV where tire inflation was important as far as going in a straight line. But filling 8 tires was a pain, so I decided to make the job easier. I would just let some air out of each tire and clamp on a chuck with a low pressure regulator. It didn't work as the pressure difference wasn't high enough for a quick fill. Is this the same thing here?

If I heat my hot tub with a small area heating element, does the tub heat faster from say 60 to 70, then 70 to 80 degrees? Because my perception has always been that it takes longer when cold.

 

[Paddy]

Industrial,

Not sure the analogy of the tires, but yes if you use a pressure reg at 32 psi, at 28 psi tire pressure fill rate slows way down. But if we look at it on the most simple type example. A block of ice will melt very slowly at 35 F. The delta T is very low. A cooler of iced beer will reach ambient of 40 F very slowly.

So back to the solar collector. Having a flow of 100 F return water passing through a 180 F manifold will transfer more heat. To use the formula we need to assign values. Sorry if there are mistakes because it's been many many years since engineering school. Let's assume 1 kg of water passes the manifold.

Q=m x c x Delta T. Where m = mass, c = specific heat and Delta T is the difference in Deg Kelvin.

c= 4186 J/kg/k ( this means water requires 4186 J of energy to raise the temp of 1 kg 1 deg C or k)
m = 1 kg This is one liter of water. Nice how metric works like that
Delta T in kelvin, deg C + 273.15 = T kelvin

100 F = 37.778 C
180 F + 82.22 C

Delta T = 44.44 K

Q = 1kg * 4186 J/kg/k * 44.44k. All units cancel except Juels : 186,034.21 J transferred

Now with 10 C difference in return temp.
Delta T = 10 deg K

Q = 4186 * 10 = 41,860 J. Transferred

So the point of the exercise shows when return water is 80 deg F cooler than the heat source, i.e manifold, 181,000 J of energy is transferred to the 1kg of water. Where as if the return water is only 10 C cooler, only 41,000 J are transferred. Heat not transferred can be lost through the insulation. Big Delta T = good heat transfer

The above shows the point of storage water temp, as Raspy pointed out as well. If you go overboard and have 2000 gal tank with just a few panels, you'll have excellent heat transfer but your tank temp might just be 85 F. Not much good. If you have too small a tank, you lose heat transfer as it approaches manifold temps and won't capture all the heat. Proper sized storage is very important. There are guides for this.

last point. Just as the return water temp Delta T vs. manifold was critical, so is the collector out put temperature. If your panel has a lower out put temperature, less heat will be transferred. This is not to say a lower temp collector creates less energy. You can have a larger flat panel making more but lower temp water and be equal to the energy of a higher temperature smaller panel.

 

[Raspy]

Industrial,

It's easy to get into a debate about storage sizing, as well as other things, but the guidelines for your particular application need to be factored in. There is more to it than equations, with solar and radiant heating. You already have some flat plate collectors that you want to modify and use, so some general guidelines, like the ones I've shared, will get you started. Then, since you are learning and experimenting, go set your system up and see how it goes. Measure it's performance and see what you get. Modify it accordingly.

Some examples of why you can't just run the numbers and declare what the outcome will be: If you want a shower every night at 9:00 PM and that's all you want from a system, then small storage is good. If you have a lot of trees or poor orientation, that affects it to, by reducing the collection window. If you want steady temps in the house then use the floor as storage in addition to a tank and go really big. To get the most out of your system you can raise the floor temp to a threshold and just top off specific zones, from storage, when needed. The larger the storage, the more you look at it as the collectors doing the work in a constant sense. Not like a gas heater where you simply turn it on when you want it, everyone wants heat delivery in a different way. ALWAYS consider the freeze protection system too, in a realistic way with every fail factored in.

After many years of designing and repairing radiant and solar systems, I decided to go relatively big with my storage and forgo quick response times. This has worked out very well. My slab, which I use for a large part of my storage, is eight inches thick and 2,800 sq ft, my water storage is 650 gallons and I have a 400 lb cast iron boiler that stores in the circuit. The gallons are small, but the mass of the floor is large. I have (4) thermostats, but have been running the whole system on one of those. (6) 4x10 collectors. In this scenario, the collectors carry the the load, not the storage at off hours, and since the floor cannot lose energy until it's warm, the rise rate is somewhat self balancing with low feed temps. It's how many BTUs you're pumping into the system that counts. Also, the storage is way too small to really carry much load for very long with this massive floor. But it works well for a quick warm up in just the bathroom in the morning. The differential temps across the floor are somewhat meaningless, as long as they are small. A small differential temperature across the solar heated radiant floor will serve you best. This is done by having a relatively high flow rate and means the floor is evenly heated. It also means the working temp of the collectors is kept low and their efficiency high.

The fun of solar is in learning by doing. Measure, compare, graph, then explain to yourself why something happened.

Anyone that declares certain systems are worthless and others are incredible, should only trigger you to see for yourself. I've built collectors, then taken them to other sights and run them alongside existing panels, with testing equipment, to prove one way or the other which method was best. And I've experimented will all kinds of radiant delivery. Once you get the basic idea, you can venture into why one style will last longer in your situation, which can be made easier and which is less likely to fail by overheating, corrosion, too much pressure or freeze.

Once the mechanical designs of the collection and storage are worked out, you can venture into control strategy. That's a lot of fun too as it involves conversations, what warm is and how it's measured as well as what methods people are currently doing to control their systems. Lots of fun.

Now you have a few guidelines and principals to help you. So get started and tell us how it goes. This is MUCH more than an armchair exercise or trying to be right about collector designs with wild claims. It's about physics, heat, cost and comfort, if you want to get the most out of it.

Here is an interesting link about tubes and plates:


http://www.heliodyne.com/industry_professionals/downloads/Evacuated Tube Comp.pdf