Solar Triple Decker
Ambrose Spencer has been going over the figures for the current state of the art of PV electricity on for example, a Boston "triple decker," three family house. As Ambrose has been thinking about and working on these issues for decades now, I'd trust his numbers.
THREE FAMILY HOUSING
In Dorchester and other parts of Boston there are thousands of triple deckers. While some midatlantic cities are filled with attached row housing, aka terrace housing which has less surface area per unit available solar estate, the predominant housing in Boston is the triple decker. These have full basements which are starting to be converted into a fourth apartment. They have three flats one over another and their floor plate is 40 to 50 feet deep from the street and the width is 25 to 28 feet. The triple deckers are in all different orientations depending on the street. The narrow face is to the street. The separation runs from 20 feet to just three feet, and is usually large enough for a narrow driveway.
For a triple decker, long dimension east and west, on the roof you can erect a solar collector 10 to 12 feet high before you start to shade the solar estate next door. Having laid out solar thermal on a couple of these I know that you can put 400 to 500 square feet before the shading next door is too much of a problem .
EXISTING TRIPLE DECKER
SOLAR ELECTRIC SOURCE 10 Mwhrs.
Using your numbers: 500 square feet at about 16 percent, 8 kw
and using 3.5 hours per day annualized is 1266 hours of collection
and 10 megawatt hours.
HEATING 17 Mwhrs
An insulated triple decker uses about 1500 gallons of oil for the three units. This is 210 mmbtu per year gross, or 50 Mwhrs net. Using a heat pump with fan coils and a cop of 3 the annual electric load for space heat is 17 Mwhrs; this does not include domestic hot water or air conditioning.
LIGHT, PLUG LOADS, APPLIANCES AND DRIVES 7.5 Mwhrs
I used 2 kw of pv allocated for a family of four. 1266 hours of collection is 2.5 Mwhrs per apartment times 3 is 7.5 Mwhrs
THREE AUTOMOBILES. 7.5 Mwhrs
I reduced my use to 10,000 miles per year at 250whrs /mile. This gives 2.5 Mwrhs times 3 is 7.5 Mwhrs. This is for battery electric vehicles.
EXISTING TRIPLE DECKER SUMMARY
On an annual basis 32 Megawatt hours is needed of which the on site solar electric, 10 Megawatt hours, can only supply one third.
IMPROVED TRIPLE DECKER
SOLAR ELECTRIC SOURCE 10 Mwhrs
The supply is the same 10 Mwhrs per year
HEATING 6.2 Mwhrs
Super insulation retrofit can cut the heat load in half, and radiant ceiling heat can raise the cop from 3 to 4. These two changes reduce the heating load to 6.2 Mwhrs.
LIGHT, PLUG LOADS, APPLIANCES AND DRIVES 5 Mwhrs
Using additional efficiency, task area illumination and higher efficiency appliances these electric loads can be cut by a third. to 5 Mwhrs.
THREE AUTOMOBILES. 7.5 Mwhrs
This load remains unchanged.
IMPROVED TRIPLE DECKER SUMMARY
The load has been reduced to 18.7 Mwhrs of which the solar electric can carry more than half
I have briefly explored a realistic application of solar electricity to the type of housing most common in Boston's older neighborhoods, in the context of a likely necessity of a 50 year schedule for accelerated climate change mitigation, although the schedule will likely be more demanding as a result of climate induced positive feedback.
I have found that to supply a typical triple decker on a Net Zero Energy basis the efficiency of the photovoltaic efficiency would need to be raised by 2 to 3 times. That is 32 percent efficiency for the superinsulated, radiant heated improved building and 48 percent efficiency needed for the unimproved building.
In a zero carbon future, all of the triple decker load not supplied on site will need to be supplied by Wind, Biofuels and other sustainable forms of renewables. Each of these carry attendant development problems like the siting of wind farms, bird kill, and the difficulties of achieving sustainable forestry on privately held lands.
Hybrid thermal electric collectors being developed by Shell in collaboration with the Dutch government and other partners can help reduce the heating load still further, but even another reduction in electric part of the heating load by one half will only affect the total electric load by 10 percent for the existing building case. .