Subject: Sunspace cogeneration

From: [email-masked] (Nick Pine)
Date: 4 Nov 1996 09:06:11 -0500

In the usual way of making electricity, about 2 kWh of heat go up the cooling
tower, etc, for every 1 kWh of electricity produced. Cogeneration makes more
efficient use of fuel, since that "waste heat" (2/3 of the fuel energy :-)
is used, and the electricity is more reliable, given a grid connection, too.
Small scale cogen makes more sense, as in the Intelligen system (about $10K
for 5 kW, ie $2/peak watt), since it's hard to move heat over large distances.
Intelligen built their first cogen system about 8 years ago, and now they have
about 80 systems in the field, which burn home heating oil. They are building
10 prototypes that will burn natural gas or propane. Their system has an 11 HP
Diesel engine attached to a 5 kW motor, ie an "induction generator." When the
house needs heat, the motor starts the engine from the grid, and the engine's
cooling water circulates through house radiators, a water-air heat exchanger,
an indirect-fired water heater, a fan coil unit, a swimming pool heater, etc,
while making the electric meter run backwards. When the house thermostat says 
the house is warm enough, the system shuts down and waits for the house to
cool off. In the winter, the electric company sends the homeowner a check
every month, or credits for summer usage. The electric bill comes out close
to $0.00 for the year, and the heating bill stays about the same, since over
90% of the energy in the oil is used to make heat and electricity.

But cogen using fossil fuels has to be an interim step, vs. a solar future,
since those fuels will become progressively more expensive as the atmosphere 
keeps getting dirtier. Combining PV and space or water heating seems like a
natural form of cogeneration, since PVs are less than 20% efficient, ie we
waste 4 kWh of heat for every 1 kWh of PV output, and PV cells cost a lot
more than mirrors. Running them hotter makes them a tad less efficient (not
much for the amorphous sort, 50 W/m^2 at 25 C vs 45 W/m^2 at 60 C), so this
is a compromise, but it can work well. Sunwatt makes a 150 W PV panel that
makes 1600 watts of water heating at the same time, using 2:1 concentration.

Putting PV panels or bare PV cells or bare solar water heating collector
plates (eg Big Fins, Zomeworks' most popular product) inside a sunspace is
another way to use the waste heat from PV or water heating to heat a house,
with sunspace air warmed by PVs or collector plates with water inside, not
antifreeze, and no pumps or heat exchangers or controls, just warm-water
thermosyphoning with a plain old 3/4" copper pipe loop to an insulated tank
or a conventional water heater or insulated upstairs, with a heating element
that rarely turns on. A simpler way to make gravity-feed hot water is to put
a few plastic 55 gallon drums on the floor above, and let warm air from the
sunspace rise up behind another layer of glazing on its north wall and heat
the drums, which are supplied with fresh water using a float valve. 

What is this PV mania? Why are a few people going bananas about the small and
shrinking electrical slice of their home energy pies, while mostly ignoring
much more cost-effective solar house and water heating systems?

Suppose we have a "superinsulated" house with average R20 walls and ceilings. 
This might be an ordinary house, with fairly airtight construction, eg a good
polyethylene film vapor barrier inside, and 5 1/2" of fiberglass insulation,
and _just a few windows_ in the walls of the house itself. Why do houses have
so many heat-losing windows? Richard Komp of Sunwatt thinks houses have so
many windows because of the popularity of early "solar houses," ironically.
Pat Hennin at the Shelter Institute thinks it has something to do with massive
magazine advertising of windows, or perhaps the FHA. Marc Rosenbaum, PE,
thinks the BOCA code requires that 4-8% of the floor space of a house be
windows. Do houses have so many windows for daylighting? Direct beam sunlight
is 200 times more intense than a brightly lit office, so if we spread it
around a room well, eg with a reflecting lightshelf below and a white ceiling
above, we only need about 1 ft^2 of window for every 200 ft^2 of floorspace,
ie about 10 ft^2 of windows in a 2,000 ft^2 house. A single 3'x4' south window,
passing Steve Baer's "sameshine" from a heliostat outside? Or a 1'x 1' window
passing a 1kW beam of 10:1 concentrated sun into the house, onto a solar oven? 

Do houses have so many windows so we can see the world outside? At night???
If not, let's put all those house windows in a thermally-isolated low-thermal
mass sunspace that heats the house proper during the day, and enjoy the
sunspace and the view during the day, and go inside and let the sunspace
get cold at night. We do not have an obligation to heat the "outdoors" :-)

I have a engineer friend who lives in a conventional house near Phila, who
has a winter heating bill of about $300. In the winter, he covers almost all
of the windows of his house on the inside with white foamboard, expanded
polystyrene coffee-cup material, which costs about 15 cents per board foot.
He just stuffs the foamboard into the window frames. A fire hazard, but he
lives with that. Malcolm Wells used to cover most of the windows in his
underground office annex with foamboard too, but he had a thin coat of nicely
stenciled stucco on the side of the foamboard that faced the office. For
south windows, we might put some dark stucco or window screen on the side
of the foamboard facing the window, and leave a 2" air gap between the window
and the foamboard, and leave a few inches of window exposed at the top, to
make those windows into passive solar air heaters in the winter. 

If we don't try to keep the sunspace warm at night, we don't need to make it
with R8 house windows that cost $40/ft^2, filled with heat mirrors and argon
gas, or even $5 R2 double glazed patio door replacement panes. We can make
a sunspace out of inexpensive single pane glass, or polycarbonate plastic
at $1/ft^2, or even polyethylene film at $0.05/ft^2, should we desire more
frugality or privacy.

Suppose our two-story R20 house is 32'x32'x16' tall. The thermal conductance
would be about 1,000 ft^2/R20 for the roof and 2,000/R20 for the walls, a
total of 150 Btu/hr-F. How much sunspace glazing would the house need, for
100% solar house heating? That depends on where the house is...

                Mon  Ta  I    I/(68-T)  Qd    SSG   SSA  Q5    ND  SCW

Prescott, AZ    Jan  36  1570    49     114K  1379   83  570K  23   8'
Albuquerque, NM Jan  34  1640    48     122K  1437   85  610K  25  10'
Phila, PA       Jan  30  1000    27     135K   774  174  675K  27  10'
Portland, ME    Dec  27   940    23     149K   691  216  745K  30  10'
Concord, NH     Dec  24   880    20     157K   618  254  785K  32  12'
Elkins, WV      Dec  32   710    19     130K   494  263  650K  26  10'
Seattle, WA     Dec  41   420    15      99K   255  388  495K  20   8'

The first column is the difficult month for solar house heating. The second
and third are NREL's average outdoor temperatures (F) and amounts of sun
that fall on south walls in Btu/ft^2/day. I/(68-T) is sun/degree days, a
measure of solar heating ease. Qd=24(68-T)150 is the amount of heat our house
needs on an average day in that worst-case month. SSG=I-6hr(68-T)1ft^2/R1 is
the daily solar gain per square foot of sunspace glazing. SSA=Qd/SSG is the
sunspace glazing area needed to keep the house warm on an average 24 hour day.
Q5 = 5 Qd is the amount of heat needed to keep the house warm for 5 days
with no sun. ND=Q5/(130F-80F)55x8)) is the number of 55 gallon drums full of
water 130 F water needed inside a solar closet to store that heat. SCW = ND/3,
rounded up to the nearest 2', is the east-west solar closet width required to
hold the drums stacked 3 deep and 2 high, in a box 8' high and 6' deep. The
size of the solar closet does not depend on the amount of sun, in this model,
just the average outdoor temperature. It is proportional to the difference 
between indoor and outdoor temperatures, ie the number of heating degree days.
The sunspace glazing area decreases with the amount of available sun. 

If the sunspace were 16' tall, with insulated endwalls and roof, the 32' south
wall of our R20 house might look like this from above, in various cities: 

                 32'                              
      12'        
|________________________________|     Concord, NH
|           |     |
|     SC    |     |
|           | SS  | 8'
|-----------      |
 -----------------
	16'

      10'        
|________________________________|     Portland, ME 
|         |     |
|     SC  |     |
|         | SS  | 8'
|---------      |
 ---------------
	14'

      10'        
|________________________________|     Phila, PA 
|         |   |
|     SC  |   |
|         | SS| 8'
|---------    |
 -------------
	12'

      10'        
|________________________________|     Elkins, WV 
|         |       |
|     SC  |       |
|         | SS    | 8'
|---------        |
 -----------------
	16'

      10'        
|________________________________|     Albuquerque, NM
|         |  |
|     SC  |  |
|         |SS| 8'
|---------   |
 ------------
	10'

      8'        
|________________________________|     Prescott, AZ 
|       |  |
|     SC|  |
|       |SS| 8'
|-------   |
 ----------
	8'

      8'        
|________________________________|     Seattle, WA 
|       |                  |
|     SC|                  |
|       |SS                | 8'
|-------                   |
 --------------------------
	24'

If some nut fills up the sunspace with PV cells, it won't make any difference
in the size required for house heating, since they are so inefficient, and 
most of the heat from the electrical energy they produce will end up heating
the house anyway. If sunspace air heats water for showers, or it contains bare
collector plates for water heating, it should be about 4' wider. If the solar
closet heats water via 16' of fin-tube pipe near its ceiling and a warmwater
convection loop, the closet should be 8' wider, and double-glazed... 

Nick

It's a snap to save energy in this country. As soon as more people become
involved in the basic math of heat transfer and get a gut-level, as well as
intellectual, grasp on how a house works, solution after solution will appear.

                                          Tom Smith, 1980