Reducing loads and adding Photovoltaic Panels (PVs)

Starting conversion to a net Zero Energy House (ZEH) Fall of 2005

ZEH December 2006

Is it possible to create a zero energy home by renovating and existing building?  Yes!  The above house was built in the 1970's.  Including the finished basement, it has about 2,400 square feet.  An energy budget of 10 kWh/day was set. A budget of $100,000 was set for the renovation.  The budget was very roughly 1/3 for solar photovoltaics (PV) and solar hot water, 1/3 for energy-related changes such as the metal roof and new appliances, and 1/3 for other changes like adding a master bath and replacing carpet with hardwood and tile.

In the first year of full time occupancy by one person the house generated over 1,500 kwh more power than it used.  So with one occupant, the home was actually better than zero energy, it generated more power than it consumed.  Beginning in  July 2007, a family of four moved into the house.  The next phase is to see if it can be a zero energy home with four people.   

As the homeowners renovated this house, the following steps were taken during its conversion to zero energy.

Reduce Cooling Loads

1. The old asphalt roof was replaced with a standing seam metal roof (the metal roof reflects heat rather than absorbs it).
2. A ridge vent was added to keep the attic cool.
3. The owners removed the plywood soffit and left the vinyl soffit. This increased attic ventilation (the plywood soffit only had 3 small vents on each side of the house).
4. Insulation baffles were installed to increase air flow through the soffit into the attic.
5. The gable attic fan was removed to prevent pressure differential between the attic and the rest of the house.

With these changes, the attic has never been more than 10 degrees above ambient. During a recent heat wave the outside temperature was 100 and the attic was never more than 110.  Adding a radiant barrier would improve performance even more.

With the metal roof and the changes to the attic ventilation, the house stays cooler in the summer.  Windows provide natural ventilation.  When the first heat wave of 2006 came through, the outside temperature was 100º F, and the attic temperature was 110º F.  Inside the house however, the upstairs temperature was 85º F, and bottom floor temperature was 76º F without any air conditioning.  The air conditioner was only used for a few days during the entire summer.  Since the first heat wave, the highest temperature of the lower floor was 81º F, and it was generally in the mid- to upper-70's.

Reduced Electric Heating Load

1. In the basement, the homeowners replaced carpet with tile to improve thermal mass.
2. One south-facing window was also added to increase natural light and winter solar gain.

Reduced plug loads

By using the meters and eliminating unnecessary appliances and phantom loads, the homeowners reduced their base load from about 500 watts to 90 watts with everything off.  On a 10kwh/day budget, still over 20% of power is being used to power things that are off!

The standby load to run the microwave is 6.4 watts. To convert to a monthly load:

6.5(24 hours/day)(30 days/month) = 4.6 kWh/month.

Most transformers use between 3 and 10 watts. Using a power strip with computers, entertainment systems, and microwave ovens eliminates phantom loads when not in use. 

CFLs use 1/4th the energy of incandescent bulbs.

Water is saved, and electricity is saved since water comes from a well that uses an electric pump.

Energy Efficient Laundry

A new high-efficiency front-loading LG Washer/Dryer combo was installed.  This Energy Star appliance saves both energy and space, since the washer and dryer are housed in the same unit.  It uses less water than standard top-loading machines for washing.  In addition, the clothing spins after the final rinse to remove most of the water from the clothes and less energy is used for drying.

On sunny days, the homeowners also make use of a convenient outdoor clothesline to eliminate the need for the dryer cycle. Line drying saves about 2.5 kwh per load. This in turn prevents burning about 2.5 lb of coal.

Shower Modifications   

Much of the warm air generated by a hot shower is lost into the room.  To minimize this effect, the homeowners enclosed the area around the top of the shower.  Keeping warm air inside the shower stall reduces the amount of hot water necessary for a comfortable shower.

Added 3kw PV

with 2.5 kw inverter

Replaced gas hot water heater with solar hot water heater

The original hot water heater in the home was powered using gas.  This was turned of when the solar hot water system (with electric backup) was installed.  Using evacuated tube solar collectors, the hot water system provided plenty of hot water through August with an average temperature in the storage tank of about 160º.  By the end of September, the average temperature had dropped to 120º.  In October, a cold shower motivated repair of pipe insulation.  In November the backup electric was turned on three times.  In December 2006 the backup electric was left on full time until March of 2007.  With the current flush mount the collector has a summer bias. The homeowners plan to tilt the collectors at a higher angle for a winter bias in the hopes of not having to use backup electricity to heat water.

Other Potential Changes

1. Replace range with induction range
2. Replace oven with convection/microwave oven
3. Install radiant barrier in attic to reduce summer attic heat and retain winter house heat
4. Put infrared reflective film on west windows to reduce summer heat gain from afternoon sun. East windows are currently shaded by trees.
5. With the windows open, the house cools to 10 degrees higher than outside morning temperature. Adding a whole house attic fan should make inside and outside temperature very close.
6. The well pump uses 1 to 2 kWh/day.  The well could be replaced or augmented with county water service.  Power used to operate an ultraviolet light (UV) to purify well water could be reduced by finding a  way to only run the UV light when the well pump is running. 
7. Replace the old air-to-air heat pump with a geothermal heat pump. 
8. Solar production in the winter can be increased by putting PV on a moveable frame that will allow adjustment for better winter performance.  
9. Add wind generation.

House energy consumption

Until the summer of 2006 this house was occupied infrequently. It as been occupied full time by one person since June 2006 and by a family of 4 since July 2007.

House = total kWh used by the house calculated by adding Grid and solar
Solar = kWh the solar panels produced
Grid = kWh used from the grid (- # means solar produced more kWh than the house used)

1 Numbers are higher than actual because the grid meter was incorrectly read. The grid meter actually moved backward for March and April.
2 Numbers are lower than actual to adjust for prior incorrect prior meter reading.
3 Average gas consumption for 2005 was 458 cu ft/month (used only for hot water). After the solar hot water system with electric backup was installed, gas use dropped to  0 cu ft/month and was disconnected.
4 Average since June 2006 when full time living at house started.
5 First full month with 4 people, some A/C was used, and the washer/dryer was used for clothing.
6 Tilted solar thermal panels for lattidue -17  to lattitude + 15
7 Tilted solar PV from lattidue -17 lattitude - 7

Solar PV was installed on 3/08/2006 when the grid meter read 2,633 kWh. As of 8/15/2007 it read 920 kWh. So the solar panels have generated 1713 kWh more than the house has used over this time period.

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