2006 Solar Home Tour

I went on the Build it Green Home Tour in Santa Clara County on October 1. It is affiliated with the National Solar Tour.

Having already had photovoltaic panels installed on our roof , and being happy with generating electricity from the sun, I went on the tour with an eye towards other energy savings ideas, particularly for heating.

On the home cooling front, we had already installed a solar attic fan , 3 Quiet Cool QC-1500 whole house fans and a retractable awning. Our photovoltaics also shade our roof a bit.

For heating, I was pondering radiant floor heating, geothermal ground loops and solar hot water. The home tour event was an opportunity to see what some local people had done in this area, and what their experiences were.

At Hidden Villa there was also a earth berm house built for the ranch manager to live in. It was a very low budget house with lots of salvaged building materials and no AC. Having earth berms built on two sides of the house helps moderate the temperature. I asked the ranch manager how cool the house stayed during the 10 day heatwave in July. He said it stayed cool in the house for the first 4 days, but with the extended hot weather, it eventually reached 85F or so, when opening up the house at night was no longer enough to cool down the thermal mass walls, floor and countertops.

In the photo you can see some columns of water. In the winter they act as a thermal mass, gathering heat from the sun during the day and releasing it at night.

Both the education center and the earth berm house were pre-plumbed for graywater. Graywater is water from sinks, showers, laundry, etc, but not from toilets, that can be reused for landscaping or other non-drinking purposes. The architect was there and he said that in San Mateo and Santa Clara counties, there are a few places which have graywater permits, but it's currently very difficult to get.

The earthberm house was also pre-plumbed for solar thermal hot water. The architect said that solar thermal was one of a number of things he wanted to do which they put off because they couldn't afford it.

At the tour they also had construction photos. The house is built within a mile of the San Andreas fault. I was amazed at the photo showing as much rebar in the walls as there was in the foundation, a double mesh.

They have both Sharp crystalline silicon solar panels and Uni-Solar thin-film technology pictured to the right. The 17 kW system is grid connected with net metering. After a 30 hour blackout, they installed a battery backup system to power a few lights and key appliances.

However, I was more focused on the question of heating. Of course they had the passive solar and the thermal mass floors and the thick insulating straw bale walls. They also had hydronic radiant floor heating. Warm water flows though plastic tubes attached to the concrete or subflooring, warming the floors which then conduct heat to the feet of occupants and also radiates heat to the bodies of the occupants. This has an advantage of more even heating than conventional radiators or forced air heating systems. Apparently it also needs less energy to keep people comfortable than forced-air heating, where you heat up the air, and the hot air rises.

I asked the building contractor how much a hydronic radiant floor heating system would cost to install under an existing hardwood floor. He said 1200 sq ft could cost $9,000 to $11,000 to retrofit. He uses a subcontractor.

I had read that you should not run the system in the summer to cool the house because of condensation. The contractor said condensation was a problem with the old style systems which used copper tubing in concrete. He said that the new systems use plastic tubing which don't have as much condensation, and the warm board around the tubing can absorb some condensation without damage. I thought the homeowner said to prevent condensation, it was best not to run the system at less than 68F.

The water for the radiant floor heating is warmed by a York E4TS060 Stealth heat pump water heater. A heat pump uses less energy than it pumps. So it would be about 2x as energy efficient as a resistive electric hot water heater or 3 times as efficient as a gas water heater. The heat pump is powered by electricity from the photovoltaic system. The building contractor said the heat pump had no trouble keeping the water temperature at 90F, and could go higher.

Before going into the heat pump, the water runs though some horizontal geothermal ground loops. This pre-warms the water slightly. They only did it because they were digging trenches anyway, so it wasn't much more to put in some pipes by the retaining walls.

The regular domestic hot water for the house is heated by a WatterSaver heat pump water heater. It is separate from the other heat pump so the radiant floor water heater doesn't need to run all year. Presumably the domestic hot water heater runs at a higher temperature of 120F-140F. The WattSaver is designed to be a "drop in" replacement for a standard 50 gallon residential water heater. I thought it might be good to get one since it ought to be more efficient than our current water heater. However when I went looking for prices, I found that it doesn't come in a 40 gallon size, and would thus be too big for our house, and also it seems they are no longer making the WatterSaver.

The heatpumps pump heat from the basement utility room or crawlspace. In the summer this has the added benefit of providing cool air from below for heat stack type building cooling.

I often hear that solar thermal is more energy efficent than solar photovoltaic. That is, a solar thermal system could take the energy that is in the sunlight and convert a larger percentage of that into heat energy in water than the percentage of energy that a photovoltaic system could convert into electrical energy. I asked the building contractor why they didn't use solar thermal to heat up the water for the radiant floor heating system. He said that they had considered it, and had chosen a site for the solar thermal panels, but that it cost too much. So instead they use the electricity from the solar panels to run the heat pump. That way it still uses clean renewable energy.

So I've pretty much decided against getting a solar thermal hot water system. Out of 3 places I visited on the solar home tour, the first place indicated that the solar hot was was insufficient. The other two places said it cost too much. How can it cost too much if solar thermal is more energy efficient and costs less to manufacture than photovoltaics? I think I know from my own analysis done when I was considering solar thermal hot water. The problem is that most of the time the hot water is not needed. During the few hours a day that you are taking hot showers, doing laundry, washing dishes, or the few months out of the year that you run the radiant floor heater, the solar thermal could very well be more efficient. The rest of the time the system sits idle. The heat gathered from the sun goes unused. You can't sell it, it's just wasted. It's better to add a few more panels to your photovoltaic system to run the heat pump to heat the water. When you don't need hot water and electricity is still being produced by the PV panels, you can sell the excess electricity to the utility company. Or to lower capital costs, you don't add 5kW of panels if you need 5kW of power to heat water a few hours a day or a few months a year. You put in maybe 1kW of panels that will give you the needed kWh of energy required over the course of a year.

Cool Residence