Tuesday, June 9, 2009

Solar green home completed and performing as hoped

So, it's been a while since my last blog. In the beginning of April, we held our open house. It was great, we had over 75 people come by and take a tour. The homeowner has since moved in and is enjoying her new home. In mid February we first fired up the heating system in the house and have been monitoring it since that time. During the first 32 days of the heating system running and the cold house being brought up to temperature, we consumed $157 worth of energy. Keep in mind this house is entirely heated by passive solar energy in combination with a geothermal heating system. The geothermal system is supplying the radiant floor slab with heat at 90-95F and maintaining the house at a very comfortable temperature even on the coldest days of February. I have to say the geothermal system is not a plug and play system. I installed data logging devices to monitor the system and the on/off cycles of the heat pump. After the first few weeks and some tweaking of the flow rates and delta T of the storage tank we finally found the right mix to make the system run at it's peak performance. What I have found is that very small changes to the flow rates can have a profound impact on how well the system runs. After all what good is it to have the most efficient heating system if it isn't working at it's peak.
We still have the solar hot water system to add to the house, we will be installing this in the next month or so. As of right now the domestic hot water is running on an electric hot water tank. Most people would think that this is not the most efficient way to heat water and I would have to agree with them, it isn't. So why would we put an inefficient hot water tank in a home designed for zero energy? I won't try and get to technical, but here are the basics needed for a solar hot water system. First you need a collector, then a storage tank, some piping, a pump or too, a controller including temperature sensors and an exchange, usually found inside the storage tank. What I have chosen to do in this case in separate the tank and the exchanger rather than combining them and here is my reasoning. Simple solar storage tanks are no different than a traditional electric hot water heater, other than a couple of existing ports to make the connections. These can be simply added to any existing electric hot water tank by adding a T at the intake and the drain port on the bottom. The cost difference is the thing, a tank sold as a solar storage tank usually costs twice as much and typically even more. What about the exchanger? Some tanks have an exchanger already inside of the them and they work excellent, but at what cost? Most of the solar tanks, that have the internal exchangers cost at least $1500 typically more like $2,000. If you separate the components and make the exchanger and the tanks separate you not only reduce cost dramatically but you also reduce maintenance costs down the road. By using and external heat exchanger you do loose some efficiency but not much. What do achieve is a system that individual components can be changed out as they fail, (somewhere down the road) without having to throw away the tank because the exchanger failed or visa-versa. Reducing the initial costs and making the system modular makes sense to me, what do you think?

The total energy cost for the house from Mid February till now has been about $400, not to bad given the cold temperature we experienced during Feb and March. So, this all electric house is ready for the next step, solar electric or PV as it's better known. For that we will have to wait, at least a year before making the decision as to the value of such a system. The cost for doing this has come way down over the last year and once the years total energy usage is determined then a system to meet these needs can be designed. all of the infrastructure for this is already in place. If my calculations are correct this home will need a 6K Pv system to meet the homes needs at a cost of about $35,000. It's hard to justify such a system if the total yearly energy costs are $1,500 as predicted by the energy modeling done before the house was even built.

I little tidbit about just how energy efficient the homes envelope system is, when compared to other homes and even some extraordinary ones. A typical home built in this area and insulated to meet the existing energy codes would use more than 2.4 times as much energy for the heating and cooling needs. Even a house designed for the Solar decathlon competition doesn't come close the efficiency achieved with this design. I am currently acting as an advisor to the BAC/Tufts University solar decathlon team, were they are currently building an 800 sq/ft home completely powered by solar energy as part of the DOE's Solar Decathlon competition. Their home, while being a great model of solar energy has a heat loss equal to my 2,000 sq/ft home. This house has a 11 Btu heat loss at design temperature. That means that the heating system for the house was so small we used the smallest one we could find to meet the heating and cooling loads of the house and it was still over sized by 30%. The maximum design loss for this house is 20,000 Btu's/hr at design temperature. That means this house will only use 20,000Btu's/hr when the outside temperature is 0 degrees and the sun isn't shining. Most homes heating systems are rated for at least 60,000 and most are more like 100,000 Btu's.

That's all for now, but stay tuned I will be shooting some new video's of the home including the new super low maintenance lawn we installed a few weeks ago. Pretty cool stuff, never needs watering once established, grow so slow you only need to mow it 5 or 6 times a year and never needs fertilizer. I hope it lives up to the expectations put out in the literature about this product, if it does I will be replacing my lawn next year. I love the idea of not having to spend a couple of hours every weekend mowing my lawn. Stay tuned.

Tom Pittsley
ecobuilder@aol.com
www.eebt.org