Passive solar home construction: Walking the walk

I have been talking the talk for a long time, but walking it is a different story.


Building a passive solar home and designing one are two different things, measuring it's performance is another. While over the years I have built several homes that were designed for passive solar all to some varying degree. In 2008 I had the chance to design and build a passive solar home for Linda Johnson and her two children, Rachel and Max. After over twenty years of construction, the last ten having been dedicated to building better buildings, I finally had my chance to use my years of knowledge as a Design/Builder to create the most energy efficient home I have ever built. It was great to work with a home owner to design the home of her dreams, while I focused on ways to reduce the energy demands and cost of the homes construction. Below is a list of the house specifications, along with the total energy usage to date.





Location: Middleboro MA

Size: 1,976 sq/ft = 183.5/sqm


style: Modified single level ranch style

3 bedroom, 2 bath, kitchen/dining/living, laundry, utility, and entertainment/wetbar


Oriented to capture as much passive solar as possible 180 degrees south along long side 68'

Roof slope maximized for active solar to be added in the future, conduit in place

Cost of construction $269,000 plus extras and appliances total cost of $290,000

$135 sq/ft base costs $145 sq/ft final cost

10% ratio of floor to south facing glazing,196 sq/ft, 46 sq/ft conventional,
150 sq/ft of specialized thermal mass windows: More info on these windows

Completely insulated slab-on-grade foundation w/integrated radiant heating/cooling R-15

6-1/2" Structural Insulated Panels for all exterior walls R-24

10-1/4" Structural insulated panels for the ceiling 1,300 sq/ft R-40, conventional frame above

7" of closed cell spray foam in cathedral roof section between conventional rafters,940sq/ft R-40

Additional insulation at sill points to address thermal bridging

Air sealing,using negative pressure during construction to find and seal all air leaks

Heat Recovery Ventilation system w/programmable control

Ground source heat pump to provide heating and cooling to the radiant slab

Energy efficient appliances

Whole house De-humidification system

All energy supplied to the house as Electricity

Date loggers to record efficiency of systems and monitor comfort levels for Temp/Humidity

The standard for total annual energy use under the Passive house Institute standards is < 120 KWh/Sqm. To meet the standard this house would have to use less than 22,000 Kwh's annually: 183.5X120=22,020.
While the house is about to undergo it's first New England winter while occupied, the performance to date has been consistent with the energy model. The house has been occupied for 6-1/2 months now and the total energy used to date is 6,020 Kwh's. If it continues through the winter meeting the expectations the total annual energy consumption will be around 12-14,000 Kwhrs surpassing the standards set by the passive institute by 33% or more. While I didn't do a blower door test to measure the air leakage rate or have a way to separate heating energy from total energy, the final Kwh's of energy consumed will be the measure. No complicated measure devises and costs, just the electric meter the electric company provides. We have been logging the energy use by simply reading the meter at different intervals and recording the actual energy consumed. On April 20th 2010 the house will have gone through one full year of occupation, on that date I will be able to confirm if it has met or exceeded the standard.

It's going to be a long winter's wait !

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

Window shading for a passive solar home

We just finished posting a video on YouTube showing how well the window shades developed to compliment our thermal mass window system work. while it was specifically designed to work with the thermal mass windows, these shades can reduce the cooling needs of any home that experiences solar heat gains during the cooling season. These are the first prototypes to be installed on a home and are functioning to reduce the cooling needs of this home dramatically. I will post more about the geothermal cooling system when I have some time, but lets just say that is far from conventional.




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

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