One Year Energy Review

The photovoltaic system has been in operation for a year as of 7/6/12, so here are a few relevant numbers. In case you want to skip the numbers and graphs, we're doing great!

• 6655 kWh were purchased from Puget Sound Energy
• 3512 kWh were produced by the PV array
• 2322 kWh were put back into the grid
• 1190 kWh produced by the PV array were used on site
• ~1000 kWh were used by the propane range

Summing up, the house used 7845 kWh in electrical energy and 1000 kWh in propane for a total of 8845 kWh. The solar array provided 45% of the electrical energy and 40% of the total energy.

How does the house compare with other houses in the US? The average US house of similar size used 29,893 kWh annually. The corresponding numbers for the western US and for Washington state are 22,683 and 24,360 kWh. Not taking the PV array into account, the house used 29% of the energy of the average US house. If we include the electricity produced by the PV array as an "internal" source of energy, the house used 18% of the energy of the average US house.

The graphs below, which are a great feature of the PSE website, show electricity usage on a daily basis for   a summer month, August 2011, and for a winter month, January 2012.
The summer data is representative of all usage except heating, and averages about 7 kWh per day. The winter usage is significantly higher because of the space heating load. Because the days are shorter, the lighting is in more use in the winter than in the summer. Note how the energy used peaks around January 18, when the low temperature was about 22 F. The third figure shows the total electricity usage for June 2012 relative to other users in the area. I'm a bit puzzled by the big difference and assume that most homeowners had to heat their house even in June.

Initial indications are that the house is behaving as expected. It's useful to compare the usage with that calculated using the Passive House software for two people. The heating component could be measured using a Belkin Conserve power meter because the heating source is a plug load - a single 1500 W radiator on wheels. The figure below shows that the measured and calculated heating loads agree quite well for the months measured. Assuming that the measured heating energy for December - April is half the annual value, the annual heating demand is 2700 kWh and that expected from the software calculation is 2009 kWh. The measured and extrapolated value is 34% higher than the calculated value. The calculated total energy is 6455 kWh whereas the measured value is 8845 kWh. This measured value is 37% higher than the calculated value.The higher values for heating and total energy could rise from structural issues such as thermal bridges that have not been modeled properly or from lifestyle issues - higher lighting and plug loads.

To Certify or not to Certify - That is the Question

The label "Passive House" is a gold star or a merit badge given to houses that meet the following 3 standards set by the Passive House Institute:

1) The house must have an airtight building shell. Quantitatively expressed, the leak rate for a differential pressure of 50 Pascal pressure must be less than 0.6 air changes per hour, measured by a blower-door test. In a passive house, the air is exchanged with outside air frequently, but in a controlled fashion using an energy recovery ventilator such that at least 80% of the heat in the outgoing exhaust air is transferred to the incoming fresh air. This standard ensures that cold air doesn't leak into the house directly. My house has a volume of 16000 cubic feet, so the standard calls for a leak rate of less than 160 cfm at 50 Pa. My current leak rate is 195 cfm or 0.73 air changes per hour.

2) The annual heat requirement must be less than 1.39 kWh/ft2/year. This standard insures that the building not use "too much" heating which requires a bit more explanation. A building loses heat due to thermal conduction through the building envelope (walls, roof, windows, slab). The heat loss depends on the insulation and the climate through the number of degree heating days and the amount of sunlight. The building gains heat by sunlight passing through the windows and through internal gains, meaning the heat the inhabitants give off as well (~80 Watts per person) as the waste heat generated by lighting, appliances, computers, etc. For my 1468 sq ft house, the heating demand should be less than 2041 kWh/yr. At the local electricity rate, that translates into $204/yr.

3) The primary energy used by all aspects of the house must be less than 11.1 kWh/ft2/yr or 16,350 kWh/yr for my house. OK, what is primary energy? Using electricity as an example, primary energy is not the energy you measure at the electrical meter, but rather the energy used to generate the electricity you measure at the meter. Primary energy accounts for the energy consumed on site in addition to the energy consumed during generation and transmission in supplying the energy to your site. Whereas requirement 2) includes only heating, requirement 3) includes both heating and cooling as well as lighting, appliances, etc. The Passive House Institute uses a site independent factor of 2.7 to convert secondary electrical energy measured at your meter to primary energy. The corresponding factor for natural gas, which could be used to heat water or provide space heating instead of electricity is 1.1.

DO THE 3 PASSIVE HOUSE CRITERIA MAKE SENSE IN EVALUATING HOUSES OVER A WIDE GEOGRAPHICAL AREA IN THE US?

The simple answer is NO. Let's look at the three criteria individually keeping in mind that the goal is to conserve energy. Each criterion should measure up to this goal.

1) The average annual temperature in Honolulu is 72F and the average annual temperature in Minneapolis is 45F. A given leak rate through the building envelope is going to cost much more energy in Mineapolis than in Honolulu, where it is zero. A more sensible criterion on leaks would link the leak rate to the sum of the degree heating and cooling days so that you are measuring the energy cost of the leak rather than putting out an arbitrary number. My house has a higher than allowed infiltration rate. What is the energy loss due to the higher leak rate? It's 82 kWh per year or a cost of $8.20 compared with an annual total energy cost of about $600. Is this a significant waste of energy?  The value of 0.6 air changes per hour is an arbitrary number that is not closely related to saving energy.

2) The heating demand is determined by two site dependent quantities: the number of degree heating days and the sunlight coming through the windows. Honolulu has zero degree heating days. You could meet the second Passive House criterion in Honolulu using paper walls. A more sensible criteria would be to combine the heating and cooling demands. By that measure, San Diego has the ideal climate with 1072 annual heating plus cooling degree days. Plus the sun shines a lot. Anyone should be able to design and build a net zero energy house in San Diego - a far higher standard than the passive house.

3) Using primary energy as a criterion is a sensible approach which includes  full cost accounting of energy production.  However, the Passive House Institute Institute assumes a SITE INDEPENDENT FACTOR to calculate primary energy. This makes no sense because these factors are different in all 50 states. Source factors for electricity generation have been tabulated by M. Deru and P. Torcellini in  NREL Technical Report NREL/TP-550-38617 entitled "Source Energy and Emission Factors for Energy Use in Buildings". The source factors vary between 7.94 for the District of Columbia to 1.67 for Oregon. Washington, where I live, has an electricity source factor of 1.74. Oregon and Washington values are low because most of the electricity comes from river water - little burning of coal or natural gas.

Finally, there is no provision in the standards for the production of photovoltaic energy. If you harvest sunlight thermally to provide hot water, the Passive House Institute gives you a gold star. However, if you generate electricity on your rooftop and feed it into the grid to offset the electricity you use to produce hot water, no gold star. Does this make sense to you?

So the purpose of this posting is to point out that the certification standards could be improved considerably. I'm a bit cantankerous and don't jump through hoops that don't make sense to me. So am I going to have my house certified? Tune in for a later blog and I'll tell you. It's not the yes or no that is important - it's how I got there.

So How is the House Working?

The goal of building a passive house was to decrease my carbon footprint, and it's time to look at the data. Historically, January is one of the coldest and therefore most energy demanding months of the year. Historically, the average temperature is 41F and the house has been held at 68F day and night. I experimented with decreasing the temperature at night, but the thermal inertia of the house is large, and the heating power is small. That combination is not well suited to dropping the temperature at night. An 2.5 kW electrical heater in the duct leaving the energy recovery ventilator was installed to provide heat. However, to get that much heat out, the ERV has to be run at fully capacity, namely 200 cfm, which is 3 times what is needed to provide healthy indoor air. At that volume, the ERV would need ~ 270 W for the blower - a waste of energy. For that reason, I've been heating the ~1500 sq ft  house with a 1500 W electrical radiator.



It provides enough power as long as the outside temperature is above ~32F. When it gets colder at night, which it did a number of times in January,  I used a second electric radiator operated at 600W to give me a boost. Because the radiator is on intermittently, I need a power meter to tell me how much electricity is being used on average. I like the Belkin Conserve, pictured above. The thermostat on the radiator isn't very good, so I used a Lux plug-in thermostat to regulate the temperature.

For the month of January, I needed 590 kW hr for heating, which translates into a average continuous power of 820 W and a cost of ~$59/month. Given that $300 monthly heating bills are not uncommon in Port Townsend, this is a good result! At this rate, my heating cost for the entire year should be less than $400. [A Seattle friend emailed me on reading this part that his bill for natural gas heating a poorly insulated older house is only $600 because natural gas in Seattle is much cheaper than electricity in Port Townsend.  He uses five times as much energy in a cold month and pays only 50% more than I do. The important number to note in evaluating the performance of the house is the kW hr for heating, not the $ cost. It may be that the only way to encourage energy conservation is to increase energy prices through taxes that are used to bring renewable energy to market.  There is a reason why hybrid cars weren't developed a decade or two earlier. Staring climate change in the face, we have begun to pay a high price for our access to cheap energy.]

So is the ERV performing? It has 2 functions: to bring fresh air into an otherwise airtight house and to extract heat from the outgoing air. I measured the efficiency of my ERV, which is defined as (Texit-Tcold)/(Tin - Tcold). Tcold is the temperature of the air entering the house from outside. Tin is the temperature of the inside air that is flowing into the ERV to have its heat extracted, and Texit is the temperature of the incoming cold air after extracted heat has been added. On a cold day, I got 0.985, and on a warmer day, 0.927. For the higher efficiency, 98.5% of the heat in the exhaust air was extracted, and the corresponding number for the warmer day was 92.7%. These numbers are higher than I expected, which I attribute to latent heat transfer. I think that the exhausted air is cooled sufficiently to condense some water and that the enthalpy of vaporization that is liberated is transferred to the incoming air.

Another function of the ERV is to get rid of moisture. Only the dryer is vented directly to the outside; there are no kitchen and bath exhaust fans. Instead, air from these rooms is sucked out by the ERV and moisture is vented to the outside in parallel with heat extraction. Although the bathrooms and kitchen have a booster switch  to bump up the ERV to its full capacity of 200 cfm to remove water vapor, we have never used this capability. Just the normal air circulation is sufficient to dry out the shower, remove water from boiling in the kitchen, etc. The Ultimate Air Recouperator is great!

My total electrical energy use for January is 1062 kW hr.The only other energy source is ~ 5 gallons/month of propane for cooking and baking. I used 472 kW hr in January for hot water, lighting, refrigeration, computers, other plug loads and my panini press. I'll track down these uses in the next blog.