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An Approach to a Zero-Energy House
Summary
For an extreme climate (both hot and cold, with little winter sun), heat loads are the
primary stressor. With little or no help from HVAC systems, the building fabric becomes
the lead system. There appear to be a couple of options to reduce heat loads to the point
where diffuse solar energy may be sufficient to keep occupants comfortable.
Stressors
Consider a climate where the winter low is –20 C and the summer high is 30 C. Both are
diurnal means, with + 5 C swings. The winter low of –25 C and summer high of 35 C are
both a bit more extreme than 99+% design temperatures for Chicago (as defined in
ASHRAE Handbook of Fundamentals).
Comfort temperatures are 20 C in winter and no more than 28 in summer.
The climate is often cloudy in winter. For design conditions, it should be considered as
cloudy.
Because the largest indoor-outdoor temperature difference is in winter, the primary
stressor can be considered to be cold winter temperatures. The absence of clear skies
means that the design will need to rely more on insulation than insolation.
Performance Specifications
Minimum heat loss through the walls and roof is key. Heat loss through the ground is
also an issue, but should be much smaller if there is decent insulation under the floor slab
or at least around the perimeter of the slab. Why? Because ground temperatures are
warmer in winter than air temperatures, due to the thermal mass of the Earth.
There will be a need for some combination of solar heat and heat from occupants and
equipment (appliances and lights) to make up for the minimal heat flow through the
building envelope. Windows should be sized to provide necessary solar heat and to
admit sufficient daylight to minimize the need for electric lights during daylight hours.
In summer, there will be a need for thermal mass to reduce indoor temperature swings
(remember the Duxford hanger, in winter) and a need for night cooling, because the
diurnal average exceeds the comfort point.
Photovoltaics or wind will be needed to provide electricity. Alternatively, one could
consider the combustion of renewable biomass, sizing a biomass farm accordingly.
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Meeting the Performance Specifications
Heating – the primary stressor, for which the façade is the lead system
Start with a windowless box, two stories. Each floor is 10 x 10 m, 3 m in height. Flat
roof. Surface is 340 m2 (we won’t worry about the ground for now – too hard, although it
would be more conservative to include it). Consider walls and a roof with a thermal
resistance of R 6 in SI units (about R 34 in English units, or about 6 inches of extruded
polystyrene, with cladding and gypsum board). The heat flow can be determined from
q  UA(Tin  Tout )
The average heat flow over a day in winter will be about 2270 Watts, extremely low for
such a large temperature difference but still a problem for a zero-energy house.
How to come up with an average thermal power of 2270 W? We have several choices:
Active solar collectors on the roof (and walls and surrounding ground, if needed)
Let’s try them on the roof, as a form of integration. We need to estimate the average
solar power incident on the collectors and their efficiency. Before going to books, let’s
use 100 W/m2 on a horizontal surface from a cloudy sky, about 10% of the maximum
we’d get from the sun (more on that as the course progresses). In winter, we’ll solar
energy 9-10 hours a day, or about 40% of the time. With a solar-collector efficiency of
50%, we’ll get an average of 20 W/m2 of useful thermal power. Over 100 m2, we get an
average of 2000 W, within about 10% of what we need.
After-the-fact note: Lechner lists 300 Btu/ft2 per peak winter day on the horizontal for
Medford, Oregon, a cloudy climate. This corresponds to about 900 Wh/m2 or 100 W/m2
over 9 hours.
Windows or skylights for solar gain
What’s the best orientation for an opening, in winter? South-facing windows take in
more solar energy on clear days than do horizontal skylights. How about cloudy days?
To keep things simple, let’s compare with the solar collectors with the same insolation,
100 W/m2 over 40% of a 24-hour day. Give the window a (perhaps wishfully high)
transmissivity of 80%. It’s taking in more energy than the solar collector, but will also
have more heat loss than an insulated wall or roof. If we cover the entire roof with
windows of R 1 in SI units, the heat loss for the house increases to 5600 W. The heat
gain from the sun increases from 2000 to 3200, not enough to make up for the increased
losses. For a cloudy day, horizontal glass is a loser.
What if we insulate the windows during the dark hours (60% of the day)? Then the heat
loss decreases to the weighted average of 2270 W (60% of the time) and 5600 W (40% of
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the time), or 3600 W. This is only 12% or so more than the heat loss, enough to make it
interesting.
After-the-fact note: Murdoch (p. 375) states that the illuminance on a vertical surface
under an overcast sky is about 400 footcandles, which corresponds to about 4000 lux or
40 W/m2. On the horizontal, the value is about 80-100 W/m2 (when the sun, hiding
behind the clouds, is about 20-25 degrees above the horizon). Our intuition seems about
right.
Photovoltaics for heat
Does it make sense to use PV for heat, as distinguished from an application that provides
power for lights and appliances? No. PV cells have a relatively low efficiency. Let’s
assume 15% for now. So we only get 600 W of electricity if we cover the roof. No
good for electric resistance heat. Even with a heat pump that produces three Watts of
heat for every Watt of electrical input, we’re at 1800 W, no better than solar thermal
collectors.
Bury the building
Earth has higher thermal conductivity than many insulating materials, but it can make up
for that with thickness. Let’s take the limit of finding a 200 m2 cave, so far underground
that the cave temperature is the yearly average air temperature, which we can take to be 5
C. This will also be the temperature of the earth surrounding the cave. Let’s build an
insulating box, with the same amount of thermal insulation. Then the heat loss is reduced
from 2270 W to 1100 W (here we include the floor). So our need for solar collectors has
been chopped in half.
An aside: are people a thermal winner or loser?
Interesting question. So far, we have taken credit for making an airtight box. Even were
that possible, would we want to do it? People need fresh air, to provide oxygen and
remove CO2. People also produce heat, a help in winter. Which side of the ledger wins?
ASHRAE, for office buildings, specifies 20 l/s of fresh air. Given the density and heat
capacity of air, that leads to 24 W/K or 960 W per person. Yikes! Another calculation
method starts with 0.5 ACH as a lower limit on infiltration, which matches 20 l/s if there
are four people in the house. A person produces about 75 W of heat, only 8% of the heat
needed to condition outdoor air. For a family of four, we have added about 3840 W,
meaning that for this very well insulated house airflow is much more a concern than
conduction.
Can we possibly come up with another 3840 W? Or can we reduce the heat needed to
condition outdoor air? A common approach – no integration here, simply an HVAC trick
– is to use an air-to-air heat exchanger or an exhaust air heat pump. In either case, the
idea is to wring heat out of air before it leaves the building. A good air-to-air heat
exchanger can recover about 70% of the heat, but requires electricity for a fan.
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Another approach is to use the walls or windows as heat exchangers, an attractive
example of integration if it can be made to work. Walls naturally work in this way, to
some extent: air that leaks in through a wall picks up some heat that would otherwise
flow out by conduction. Facades in high-tech office buildings are engineered in some
cases to bring air in through a circuitous path, to pick up heat flowing out.
Another aside: where does structure come into play?
For heating, structure serves two purposes that express an integration with HVAC. Put
another way, we want other building systems to provide heating in lieu of a dedicated
HVAC system. First, we need exposed mass to soak up heat on rare sunny days, to
prevent overheating and ease the burden on solar collectors. Second, we do not want
structure to form thermal bridges, which will increase heat losses. Post-and-beam
construction with stress-skin rigid-foam panels are one solution, because the panels
entirely cover the structure.
Electricity for lights and appliances
Here’s where we need to consider PV or wind. Consider a winter day, with minimal
daylight hours. Let’s say we need lighting for 60% of a 24-hour day, at 10 W/m2 for half
the house or 1000 W. For the remaining 40% of the time, we’ll hope we get enough
daylight. (We can estimate this, taking it beyond wishful thinking.) The average lighting
power is 600 W. Too much! In a house larger somewhat larger than 200 m2, monthly
electricity usage might be about 400 kWh for everything (lights, fridge, washer, electric
dryer). This is an average of about 550 W. For a zero-energy house, it is reasonable to
expect to use the most efficient lamps and appliances. Maybe we can get down to 400 W
on average. Covering 100 m2 with photovoltaics could provide about 600 W, as noted
above. The extra could be used to power a fan needed to push air through wall or
window cavities or a dedicated air-to-air heat exchanger.
Note that the 400 W of electricity is dissipated as heat, which reduces what is needed
from solar gain (through windows or via solar collectors).
Can windows or skylights provide enough light to eliminate the need for electric
lighting? One rule of thumb for commercial buildings is that daylight should be
acceptable within 5 meters of windows. Clearly depends on the size of the windows.
We’ll learn later how to calculate this.
Another approach is to take the 100 W/m2 on the roof and convert it to lux, or lumens/m2,
using 100 lumens/Watt. We then have 10,000 lux outside the glass and say 8,000 inside.
If we want 300-500 lux and we lose about 50% to absorption by room surfaces, we would
need 5% roof openings for a single-story house and 10% for two-story (plus a means of
piping light down to the first floor). So using at least some skylights, insulated at night,
would appear to be a good idea because they bring in light as well as heat.
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We have neglected cooking. For now, live on peanut-butter sandwiches. Can think
about generating more electricity or using a solar cooker or biomass or simply give up.
After-the-fact note: Baker (p. 45) shows that we can get a daylight factor of 2% (2% of
outdoor horizontal illuminance indoors) about 3 meters in from a wall with 30% glazing.
For an outdoor illuminance of 10,000 lux, we would get 200, marginally acceptable.
Daylight will not be sufficient further from the window, unless the window is larger.
Cooling
For the climate chose, cooling is a bit challenging but less so than heating and electricity
for lighting and appliances. At first glance, it’s hopeless: the diurnal average outdoor
temperature exceeds the specified upper bound on thermal comfort And solar gain, heat
from occupants, and heat dissipated from lights and appliances only make things worse.
But there’s a trick.
The occupants can open the windows or doors and hope that wind-driven airflows are
sufficient to bring the indoor temperature down to a level close to the outdoor
temperature. But this is not a good idea in the heat of the day, when the outdoor
temperature rises to 35 C.
The occupants could shut up their house entirely. If the house were sufficiently massive
(here’s the integration of structure and HVAC again) and if there were no internal gains,
the indoor temperature would vary little over a day and would equal the outdoor average
temperature. But with 400 Watts of electricity and 300 Watts from sensible body heat,
the indoor temperature would rise 5 C (which can be easily calculated with the same
heat-flow equation shown above).
Can the smart occupants combine the two strategies? Yes!! They can open up at night,
when it is cool, and shut up in the hot day. What’s the limit to this? Clearly, it cannot get
cooler inside than the outdoor minimum, which is 25 C. That value may not be practical,
but it is reasonable to try to reduce the indoor peak temperature to a value lower than the
diurnal average outdoor temperature, 30 C.
Here’s a way to estimate what can be done. Imagine cooling down the structural mass or
dedicated thermal mass (tubes filled with water, for example) to 25 C at night. Over a
15-hour period with the windows closed, heat from lights and occupants (700 W total)
amounts to 10,500 Whr or 37.8 MJ (3600 J equals 1 Whr). If we want the mass to heat
up to 28 C and no more, how much do we need? If water, with a heat capacity of 4200
J/kg K, we would need 3,000 kg. The density of water is 1,000 kg/m3, so we need 3 m3,
an achievable amount. Concrete in building mass should also work.
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