Frequently Asked Questions

 

Solar Thermal

 

 

Can solar energy heat 100% of my domestic hot water AND still heat my home?

Yes, but most people in the northern regions probably wouldn't make the capital investment to do so. Certainly, with enough flat plate panels or evacuated tubes virtually any home could be heated for, at least, most of the winter (domestic hot water with that many panels is easy). Especially if the home has good insulation and a good solar orientation to start with. But in practical terms, few homeowners enjoy a perfect solar profile (good insulation, south facing orientation, modest square footage to heat), and few have the desire to invest in a system so huge it has virtually no hope of payback.

The problem is consumption. Because any home in the northern tier states, at zero degrees F., for example, needs massive amounts of heat to maintain a 68 degree indoor comfort level, a high BTU fuel source is required. Solar, though free (once the equipment is paid for) is a "low density" heat source and can never really compete with the massive, stored energy density of, say, oil or natural gas. Why? Because even without cloud cover, a solar array only receives about six useful hours of sunlight during the coldest months of winter. Too bad the heating day averages about twenty hours a day.

Also, a system designed for 100% solar heat in a far northern climate becomes vastly oversized during the summer when domestic hot water becomes the system's primary job. In this case, an elaborate "heat dump" or a large swimming pool/hot tub is required to "divert" the excess heat and protect the system. Follow this link for more details on the solar heat dump.

 

What is the most practical application of solar water heating?

 

Realistically, finding a balance between upfront cost and overall heating requirements. Although impractical as a singular energy source in the colder climates, solar can still provide an abundance of free energy. Sure, maybe solar can't "do it all", but investing in an appropriately sized system (one with a reasonable payback period) makes tremendous sense.

And, of course, if you're using solar in New Mexico, Arizona, Colorado, or any reliably sunny location, solar can provide 90 to 100% of all heating needs. Especially if the home is well insulated and oriented to benefit from passive solar.

 

How much equipment will I need?

 

Again, as outlined above, realistically assess your individual solar needs. How much are you willing to spend? What percentage of your space heating requirements do you want solar to provide? Can your collectors receive maximum solar gain, or are they shaded during part of the day? Are you trying to heat a small, well insulated cottage, or a leaky 5,000 sq. ft. 1890's farmhouse? Do you expect solar to provide most, or all, of your domestic hot water with only a modest contribution to space heating? Or are you determined to size the system to heat domestic hot water AND provide space heating?

Answering these questions will help you focus on realistic equipment needs.

 

Which is better: Flat Plate Collectors or Evacuated Tubes?

 

In regions with lots of sun, the performance differences between flat plates and evacuated tubes narrows. In colder regions with lots of winter cloud cover, the evacuated tubes perform better. Unlike a flat plate collector, an evacuated tube converts 92% of the solar radiation it receives into thermal energy. This is because once the heat enters the tube, it can't escape through the vacuum. Though fairly well insulated, the body of a flat plate collector absorbs heat and radiates much of it back to the surrounding air.

 

What is the lifespan of the collectors?

 

A flat plate collector is basically an insulated metal box with copper tubing running through a copper or aluminum absorber plate. It's lifespan will be comparable to that of household copper plumbing.

An evacuated tube collector uses "heat pipe" technology to transfer solar energy from an absorber plate to a collector header. This absorber plate is contained in a sturdy borosilicate glass tube designed to withstand hail up to 1 1/4" in diameter...so breakage really isn't a problem.

Probably its weakest link is the vacuum within the (evacuated) tube itself. Tests have proven that this state of vacuum endures for about twenty years. But even without a vacuum, the components, (i.e. absorber plate, antifreeze, heat pipe, condenser bulb), will still produce heat. I know. I've stood in the sun with my hand on the condenser bulb of a blown (lost vacuum) evacuated tube. After about two minutes, the bulb became too hot to touch.

Technically, the evacuated tube collector should function at 75% (my guess) of its rated efficiency for fifty years...or until the glass tube physically breaks somehow.

 

How much do solar water heaters cost?

 

As always, there is no simple answer, just more questions.

For example, how big is your house? How cold does it get in your region? How much hot water do you use? What percentage of heating do you expect solar to provide? Do you have an excellent solar location, or rows of ancient maples shading your panels in the summer?

You can see how this question can only be answered on an individual basis. But, I can invent an example that you can compare to your situation.

Assuming a reasonable solar orientation, and no extraordinary heating or domestic hot water needs, a 32 tube array of evacuated tubes will provide supplemental space heating (about 25%) for a well insulated 2,000 sq. ft. house, and at least 75% of all domestic hot water needs for a family of four.

Remember, there could be periods during the summer, weeks perhaps, when the system provides 100% of all the domestic hot water. But, estimating conservatively, and allowing for rainy periods and times of extraordinary hot water usage, 75% is reasonable.

Also remember that the above estimate applies to northern New England...not the sunniest location in the world. My brother in the high desert of southern Arizona (temperatures do reach the lower teens) receives 95% of ALL his space heating AND hot water from solar using a system pretty much like the above example...and he heats over 2500 sq. ft.

So, with all this in mind, a 32 tube array of evacuated tubes, including all mechanical components like heat exchanger, expansion tank, air eliminator, and electronics (but not including a solar storage tank because often that item is purchased locally or is already owned) costs about $4,700.00. Depending on the many factors listed above, and how fast fossil fuel prices rise, the payback on a system like this will be somewhere around 5 to 15 years.

Substituting flat plate collectors (three 4 X 10's) for the evacuated tubes, the same system costs about $3,700.00, but is less efficient.

 

 

Off-Grid Radiant

 

Because photovoltaic panels perform more efficiently at colder temperatures and solar reflection off snow helps offset the shorter winter days, even the small 1,280 watt solar array shown above can power this 2300 sq. ft. geodesic dome. During extended periods of cloudy weather, however, a secondary charging source is needed. This can be wind, micro-hydro, a fossil fuel powered generator, or in the case of a "grid-tie" system, the power grid itself.

 

Can a radiant heating system be powered with solar energy?

 

Yes. Modern inverters produce a very clean, efficient 110-volts from a DC battery bank charged by an alternative energy source. Radiant circulators are very low wattage pumps (about 60 watts) and, although they can have a fairly long duty cycle (hours run per day), a reasonably sized photovoltaic system can power them easily.

 

I don't have an inverter. Can I power a radiant system with an efficient low voltage DC circulator?

 

It's possible, but expensive. A pump called the El Sid uses only 10 watts of DC power to run a circulator. It's powerful enough to pump water through about 300 linear feet of 7/8" PEX. That's a pretty small zone. A 2,000 sq. ft. home would require five El Sid's. At $250.00 each, the payback period would probably exceed the life of the pump.

You'd probably be better off buying an inverter and running standard 110-volt circulators.

 

I have an outdoor wood boiler. The manufacture says I should run my boiler circulator 24-hours a day. Is this practical with an off-grid radiant heating system?

 

The issue here is "duty cycle". Any device or appliance that runs 14 to 24 hours a day is considered extreme for an alternatively powered home. A high wattage appliance like a well pump may consume a lot of power, but its duty cycle is short...unless you're irrigating a golf course 12 hours a day.

Normal domestic water use is insignificant from a duty cycle standpoint. The same is true for other high consumption appliances like washing machines, clothes dryers (gas), vacuum cleaners, etc. But a low wattage device, i.e. lightbulb, high-efficiency refrigerator, or radiant circulator can easily out-consume a washing machine if its duty cycle is long.

In the case of an outdoor wood boiler, the manufacturer recommends a 24-hour duty cycle for the boiler circulator because water in the boiler jacket (as much as 400 gallons in the larger boilers) stratifies when it's heated. In other words, without water circulating constantly between the boiler and the boiler's heat exchanger (located in the house), the hottest water, say, the top 250 gallons, reaches the high limit temperature (185 degrees) and the boiler closes down. This may leave the 150 gallons at the bottom of the boiler jacket considerably cooler. So, in effect, when the heating system calls for hot water, the circulation pump activates, the hottest water at the top of the boiler mixes with the cooler water at the bottom and the net result is 400 gallons of 145 degree water instead of the 185 degree water you really want.

This mixing effect may not matter if your boiler is somewhat oversized. But, if you purchased a 500,000 BTU boiler because you really need its full heating capacity, you're not really getting it.

Then there's freeze protection. Many boiler manufacturers recommend running the boiler pump 24-hours a day to keep hot water flowing through the lines. This sells boilers by making installation easier. Because outdoor wood boilers are often installed thirty, one hundred, or up to five hundred feet away from the house, the expense and effort of digging a trench below the frost line becomes a real issue. But if freezing isn't a problem (because hot water circulates constantly) the boiler manufacturer can recommend a shallow trench.

So, where does this leave off-grid wood boiler-fired radiant installations? The quick answer is "as close to the house as possible".

If your region freezes hard in the winter, even insulated supply and return lines are vulnerable in a shallow trench. In the short term, frozen lines leave you without a heating system when you need it the most. In the long term, ( the following summer, for example) imagine digging up a seventy-five foot (or longer) trench in search of a damaged water line.

Basically, unless your pipes are safely below the frost line (3 to 4 ft), the hot boiler water should flow continuously.

Of course, anti-freeze is a solution, but a very expensive one (remember the 400 gallons in the boiler). Not only initially, but every three or four years a fresh 50% water/anti-freeze mix must be added to the boiler's water jacket. For -29 below zero protection, (at $15.00/gal.) expect to pay $3,000!

Okay, let's forget the anti-freeze approach.

An off-grid home can't live with a 24-hour duty cycle either.

Even the normal duty cycle of a standard radiant system challenges an off-grid home, but 12-hours of pumping is obviously better than 24. So, install your wood boiler as near to the house as possible. This diminishes not only the length of a very deep trench, but also the quantity of water sitting in buried pipes and cooling down between heating cycles. Dig your trench below the frost line, and keep the boiler fully stoked with wood at all times. Fully stoked? Yes. Because the idea is minimal energy consumption. The wood boiler wants to heat that 400 gallon water jacket to 185 degrees. If the furnace isn't full of wood, the damper solenoid remains in the "open" position, trying to supply the furnace with a supply of combustion air. But obviously, without a supply of wood, no amount of air will heat the water jacket. This open damper then becomes another long duty cycle draining your batteries.

Fortunately, radiantly heated homes enjoy a very long thermal swing. In practical terms this means that, even though the circulator between your boiler and your radiant system only runs when the heating system actually calls for heat, and that between heating cycles the water outside in the supply and return lines will cool down...in spite of this, the temperature of the living space drops so slowly that the "lag" between the thermostat calling for heat and the boiler's ability to deliver it won't be noticed.

So...for off-grid applications:

1. Short supply and return lines buried below the frost line.

2. Fully loaded burn chamber to keep water temperature high, thus minimizing the duty cycle of the boiler's damper solenoid.