Solar Thermal
Submitted by admin_green on Fri, 09/11/2009 - 14:00
We knew we were going to get into hot water sooner or later.
But sometimes, hot water is a good thing. Like in baths and showers on a January morning in Wisconsin. Or in the jacuzzis that are such an integral part of the B&B experience.
This is an evacuated tube system, one of the systems we will consider. Each glass tube has a smaller copper tube inside. Solar heat is concentrated there, passed to a thermal exchange fluid which flows through the white pipes, and transferred to the water for the house in a boiler inside the house.
While most people think about solar voltaic panels first when they think about solar power, a solar hot water system will usually have the most immediate impact on your energy use. Solar thermal systems are more efficient than solar electric (it's easier to capture heat than light from the sun), cost less to install, provide 60-95% of your hot water use, and generally pay for themselves in 10 years or less (this article does a very good breakdown on home systems...published statistics are all over the place, so we found it very hard to nail them down for this article.) They can also be a very economical addition to a mortgage, as explained here.
The Department of Energy has a very good introduction to solar hot water systems. Wikipedia's entry is also very good. As with any renewable energy installation, you should be sure to the the DSIRE web site for a complete list of current government incentives for renewable energy.
It is generally recommended that you install a solar thermal system before you install a solar voltaic system. The idea is to reduce your overall energy demand before you plan the size and scale of your PV system . Considering that hot water consumes at least 13% of home energy use, that's a nice reduction.
This is a flat panel solar collector, shown next to a set of photovoltaic cells.
In fact, Hawaii will require all new homes to have built-in solar thermal systems beginning in 2010 (you can read about that here.) In Israel, 90% of homes have solar hot water systems installed (you can see some great pictures here...what a sight!) China is now heavily involved with solar hot water production, with about 80% of world production capacity now coming from there.
Wisconsin, however, is not Hawaii or Israel. As this map shows, we're among the least solar-friendly states in America. Nonetheless, solar thermal will work very well for the Inn. According to our solar assessment, we should be able to generate 50-60% of our hot water from a solar system (see our assessment).
This map shows solar resources for the United States. Lake Geneva falls in the 4.5-5.0 kWH/Day region, so we've got our work cut out for us. Click on the map to see it at full size over at NREL, where it was created.
Tom Krawczyk (you can contact him here), who also did our home energy audit, did a solar assessment for the Inn site on June 9, 2008.
It would have been neat, from a teaching standpoint, to incorporate a direct circulation hot water system into the design of the Inn. In these kind of systems, the water passes through the solar collectors and is heated directly by the sun. In some cases, the water tank is connected to the panels, and the water circulates passively, with the heated water rising into the tank while fresh, unheated water sinks into the collector.
This monobloc solar hot water system features four panels and an integrated water tank. Cooler water settles into the panels; as it is heated by the sun, it rises back into the tank, displacing cooler water. Very efficient; very nice. But in Wisconsin, not feasible. Click to see a larger image.
Visually, they're great. They are also the least expensive and most efficient solar hot water systems. But in Wisconsin, with our long winters and sub-zero temperatures, they're simply not feasible. We'd be constantly worried about water freezing and pipes bursting. So instead, we'll use a heat transfer system, where a secondary fluid (essentially, antifreeze) is heated by the collectors and transfers the heat to the water in a tank inside the insulated building.
Also on the plus side, indirect circulation systems require less reinforcement for the roof.
We'll be considering both evacuated tube and flat panel collectors (see the pictures at the top of the page).
The landscape challenge.
Humans love trees. The gentle murmur of wind through the leaves. The changing colors signaling the passing of the seasons. The gnarled bark telling a history that may go back hundreds of years.
Solar panels hate trees. They block the sun. End of story.
This is one of the challenges you face in creating a structure designed to enhance both the human experience and efficient use of renewable energy.
Google Map view of the current house showing proposed location of solar hot water panels.
For solar energy, there is one direction that matters, and that direction is South. The current house has ample roof space facing directly south, so there will be plenty of room for the panels needed to heat our water. But we had concerns about how the current landscaping might affect hours of sun. As the panoramic shot below shows, we are lucky to have a substantial stand of old-growth trees directly to our South. Right where a solar panel DOESN'T want them. Would we have to consider a tradeoff between the environment and efficiency?
The Solar Pathfinder readout shows hours of sunlight throughout the year based on location of the panel and natural obstructions to sunlight. Numbers running left to right on the bottom are hours of the day (5am through 7pm). Curved lines crossing the field are months of the year. They're not sequential: instead, they run (top to bottom) from lowest sunlight months to highest sunlight months. The black blob is time when the sun would hit the panels.
As the readout shows, there will be a bit of an impact from the large tree located to the East (that's the "bite" taken out of the right side of the readout), but not much beyond that.
Luckily, as tests showed, the trees are set far enough from the house that they won't significantly effect hours of sunlight. The angle of the panels, set to catch maximum exposure, will pitch them mostly above the obstruction of the trees.
Just another of the challenges of trying to retrofit and existing house and grounds to a new set of expectations. And further proof that it's all about the angles.
What about the other 40%?
So, what will we do when the solar system isn't providing enough power to heat our water? One advantage of solar systems is that they preheat the water in the system, even if they can't bring it to full hot-water temperature. That decreases the energy demand even when you have to fall back on conventional heating sources.
There are several other possibilities we are considering. One is a tankless approach, which heats only the water passing through the pipes directly before it passes into use. The other is a "smart" water tank system. America is one of the only countries where water heaters work round the clock. We spend a lot of money keeping our water at optimum temperatures at 3 in the morning! A system set to heat water only during times of expected use (it only takes a few minutes to bring a tank, and especially a preheated tank, to full hot-water temperature) would cut down on demand substantially.
We'll let you know what we discover.
