Important New Solar Tank from Bradford White

Bradford White now has a natural gas backup solar tank available for single tank SDHW systems.

Here's the description:

http://www.bradfordwhite.com/images/shared/pdfs/specsheets/507-B.pdf

This means that the old solar rule "You must have a two tank system if your backup is a gas fired tank-style water heater" is now wrong.

Single tank systems ALWAYS win the efficiency and cost contests in every report I've ever seen.  There is less area for heat loss, less dollars spent on tanks, less real estate occupied.  

But here's the most important (but least obvious) advantage of a single tank system-- if there is solar heat available, it rises up to the portion of the tank that is normally heated with the backup source.   Now the heat loss of the backup tank is made up by solar, not gas or electric.  In a two tank system, a recirculation pump or thermosiphon would be required to accomplish the same thing.

In theory, I like this new tank.  It works simply by placing the temperature sensor higher up in the tank.  That means the lower half of the tank will be cool until solar is available.  What seems  a little magical is how does the heat reach the top of the tank without heating the bottom of the tank, since the burner is still at the bottom?

Well, in an 80 gallon tank, let's just say the burner has heated the top 40-60 gallons by 6am.  Now everyone in the household takes their morning shower and uses 40 gallons.  Since the thermostat is high up in the tank, it doesn't see the new cold water that is now in the bottom of the tank.   The burner doesn't come on, and solar will heat up the bottom of the tank, and if there is excess solar heat for the day, it rises up and helps keep the burner off all night as well.

This latter effect is significant for tank-style natural gas water heaters, because they typically lose 30% -43% of their heat per day to the room they are in.  You definitely want to replace that standby heatloss with solar if possible.  Single tank design is the easiest and best way.

There is one missing piece to the puzzle for winter and low solar days.   If the top of the tank finally drops below the thermostat setpoint, the burner will fire until the thermostat is satisfied.   Since the heat reaching the thermostat must travel through the lower part of the tank, the “solar” section of the tank will be hotter than necessary.   Hot water in a solar tank always hurts solar efficiency.   B-W doesn’t report by how much, and I think somebody needs to find out.   Otherwise, solar curmudgeons will never be convinced to use this tank.

Matt Carlson of Sunnovations is also recommending single tank systems:
http://www.sunnovations.com/sites/default/files/Sunnovations_Newsletter_August2010.pdf

Passive Solar DHW

To some, the holy grail of solar thermal is a system without pumps and controls.

Himin of China manufactures a nice one:

At Solaron in the '80's, we purchased the manufacturing rights to a passive DHW design developed by Arthur D. Little. The problem was that the pipes going to the collector would freeze in the winter. Frozen pipes in the attic were so undesirable and difficult to eliminate that we ultimately abandoned the whole project. As far as I know, no one is pursuing this type of design in northern climates.

Now that building codes are beginning to actually recommend insulated roof planes and conditioned attics, the Chinese design looks very interesting. The pipes in the attic won't freeze because the attic is inside the house envelope. As long as the piping to the roof-mounted tank is kept extremely short, it won't freeze either. Natural convection within the pipe and heat conduction along the pipe from the tank and the attic area would prevent freezing.

Note that the water in the evacuated tubes probably won't freeze.  Every day they warm up a bit even if the day is completely overcast.  This insolation should be enough to prevent freezing even under the worst conditions in Denver.

From Arthur D. Little, we also learned that the roof-mounted tank will never approach freezing temperatures anywhere in the continental US. (Basically there's enough solar insolation even on completely cloudy days to keep the tank above 40 degrees)

The main attraction of this design is that system efficiencies are far superior to pumped designs. Also, with no moving parts to fail, the life-cycle cost is held low, and the ROI is maximized.

Edit, corrections 1/10/11 (the tubes contain water, not heat pipes.)

Domestic Hot Water for Zero Energy Homes

Zero energy homes (ZEH's) in the near future will be simple, all electric, and supplemented with PV.

Hybrid PV/thermal solar collectors may be the best solution as costs come down. Sun Drum Solar is now offering a product. Dawn Solar also offers a building integrated hybrid PV roofing system. PVTSolar uses an air-based approach. When I worked at Solaron in the 80's, we abandoned air-based systems, and we were the world leader in air-based until that point. These are the only entries so far in the hybrid market sector. I expect to see more manufacturers enter the fray soon.

Whether or not a solar thermal DHW system is used, a grid-sourced backup water heater is required.
An affordable heat pump hot water heater (HPWH) is now on the market:
https://airgenerate.3dcartstores.com/index.html

This type of system uses only 25%-35% as much electricity as a standard resistance water heater, making it the best choice for a ZEH without solar thermal.
Another bonus of these heat pumps is that they can supply "free" summer cooling for the house.

In the near future as costs stabilize, we'll learn if this $700 is worth it for a home with solar thermal or hybrid collectors. In Denver, a home with 100-150 ft2 of solar thermal or hybrid collectors probably won't need it. The $700 marginal cost would take too long to recoup because the solar is providing 90%+ of the domestic hot water load.

H4H Redefined the State of the Art for Zero Energy Homebuilding

With NREL, Habitat for Humanity has built and tested the simplest Zero Energy home that I've seen to date. Lots to learn from the following final technical report:

http://www.nrel.gov/docs/fy08osti/43188.pdf

Perhaps the most surprising lesson is the poor overall system efficiency of the solar thermal DHW preheat system. It shows that these systems may be hardly worth the effort. Ongoing maintenance for a system like this will further degrade the cost/benefit ratio for this subsystem.