Saturday, June 26, 2010

Getting to Zero

The average Canadian single family dwelling uses an unfortunately high total of 238.9 kwh/square meter of house area.  For our 180 square meter house (1900 sq ft), this would equal 43,002 kwhrs/year broken down as follows:

 Household Energy Use
Use Consumption %
space heating 28613 kwh/year 67%
hot water 6663 kwh/year 15%
appliances 4942 kwh/year 11%
lighting 1948 kwh/year 5%
space cooling 835 kwh/year 2%
Total 43002 kwh/year

There are really only two practical ways to make energy on a tight lot in the city in Ottawa– solar photovoltaic (electricity) - called PV, and solar thermal (direct heat from the sun).    There are other renewable options for space heating – like burning bio-mass (wood, grains, waste etc.) that can help with space and water heating, but electricity that meets the “zero carbon” concept pretty much needs to come from the sun given all the constraints in the city.

 The PV technology on the market today varies in terms of efficiency to turn sunlight into electricity. A good German panel that is roughly 1.63 square meters (17.3 sq.ft), has a nominal output of 230 Watts.   “Nominal output” means that when the panel is brand new, and in direct sunshine it will produces 230 Watt hours in an hour.  On average throughout the year, Ottawa gets about 3.8 hours of useful sunlight per day that can be turned into electricity.  Not all that electricity can be used though, first it needs to be inverted from DC to AC and run through a meter etc.  This can add up to 25% in losses.  After some math, we found that for each kW of nominal output on our roof, we can generate about 1037 kwh/year of electricity.   When you string the PV panels together to make an “array”, you need to add some space between panels for clearance and venting plus space for wire runs.  We have found that we require around 8.5 square meters (90 sq. ft.) for each kW of nominal outuput.

So.........a typical house of our size uses 43,002 kwh/year
Divided by  1037 kwh/year output per kw of nominal capacity
Equals 41.47 kw of required capacity
Multiplied by 8.5 meters per kw
Equals 352.5 square meters (3746 sq ft) of south facing roof space.

The cost for this system would be higher than the renovation budget for the entire One Planet Reno project, and our entire lot is only 242 square meters (and a good chunk of that is in the shaded backyard which would be almost useless for PV).  The roof is just 640 square feet, of which 20% of it is shaded in the spring/fall.  Needless to say, this is not an approach that can work on our project.  This helps explain why all projects that attempt zero carbon, or net zero energy, must drastically reduce the energy demand first, and then turn to renewable energy production. That is why we are using the Passive House approach to energy efficiency and leaving the beer fridge behind at our current house.

We will post our projected energy profile later, once we have the solar thermal system designed.

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