someone else

1
someone elses house
2
Heating and cooling a building
The average temperature difference between the
inside and outside of the house depends on the
setting of the thermostat and on the weather. If
the thermostat is permanently at 20 C, the
average temperature difference might be 9 C.
The leakiness of the building describes how
quickly heat gets out through walls, windows, and
cracks, in response to a temperature difference.
The leakiness is sometimes called the heat-loss
coefficient of the building. It is measured in
kWh per day per degree of temperature
difference. efficiency of heating system is
usually the percentage for the conversion of
input energy (e.g. gas) to output energy of the
system here it is a fraction (i.e. 90 ? 0.9)
3
Example the leakiness of my house in 2006 was
7.7 kWh/d/ C If the average temperature
difference average between the house and the
surrounding air is 9 C, the heat loss (rate at
which heat flows out of the house by conduction
and ventilation ) is 9 C 7.7 kWh/d/ C ?
70 kWh/d to calculate the power required, we
divide this heat loss by the efficiency of the
heating system. in this case the condensing gas
boiler has an efficiency of 90, so we find the
power used is 70 kWh/d / 0.9 77 kWh/d
4

• To improve the system
• Reduce the average temperature difference learn
  to live with less heating/cooling
• Reduce the leakiness of the building by improving
  the buildings insulation think triple glazing,
  draught-proofing, and fluffy blankets in the loft
  
• or, by demolishing the building and replacing it
  with a better insulated building,
• or perhaps by living in a building of smaller
  size per person (this means smaller square foot
  floor space)
• 3. Increase the efficiency of the heating system.
  You might think that 90 sounds hard to beat, but
  actually we can do much better.

5
Cool technology the thermostat The thermostat
is hard to beat, when it comes to value-for-money
technology. You turn it down, and your building
uses less energy. Magic! For every degree that
you turn the thermostat down, the heat loss
decreases by about 10. Turning a thermostat
from 20 C to 15 C would nearly halve the heat
loss.
Thanks to incidental heat gains by the building,
the savings in heating power will be even bigger
than these reductions in heat loss.
6
Heating is personal Preferences of the occupant
or commercial interests are important. Figure
21.2 shows data on the heat consumption in 12
identical modern houses. Family 1 consumes 43
kWh per day - twice that of Family 12 But DMs
house consumes 77 kWh per day so what is going
on? The problem is the house. All the modern
houses in the Carbon Trust study had a
leakiness of 2.7 kWh/d/ C, but DMs house had
a leakiness of 7.7 kWh/d/ C!
7
And in a university (University of Central
Florida) Does the university have a standard for
indoor temperatures?Yes, during cooling seasons
70-74 F and during heating season 6870 F.
Comment These are 22 C summer, 21 C
winter. Lessening the difference by 5 C means
turning down the thermostat in winter to 16 C
(61 F) and turning the thermostat up in summer
to 27 C (81 F). The EPA suggests 71 F is the
optimum productivity temperature for
buildings What is the connection between
humidity, temperature and comfort?There is a
complex relationship between humidity,
temperature and individual comfort
(psychometrics). Air movement and radiant
temperature (from exterior glazing) have large
influences on personal comfort. How can I get
more comfortable with the temperature?If the
temperature is within standards, you can dress in
layers, keep a sweater, jacket, or lap blanket in
the office and be prepared to add or remove outer
layers of clothing for your personal comfort.
http//www.pp.ucf.edu/operationalservices/hvac/ind
oor_temperature_faq.html
8
Large and easy Improvements Live in a terraced
house (old style urban America and Europe) with
modern renovations
9
Authors history of savings (SLU take note)
Figure 21.4. Domestic gas consumption, 1993 to
2007. One line for cumulative consumption during
one year in kWh, end of year average is indicated
in kWh per day. Meter-readings are indicated by
the blue points. Evidently, the more frequently I
read my meter, the less gas I use!
10
Renovations standard in new homes
11

• Beat down the costs its wartime!
• Install condensing boiler, replacing the old gas
  boiler. (Condensing boilers use a heat-exchanger
  to transfer heat from the exhaust gases to
  incoming air.)
• Remove centralized hot-water tank (so hot water
  is now made only on demand),
• Put thermostats on all the bedroom radiators
  (each room).
• consumption decreased from 50 kWh/d to about 32
  kWh/d
• insulation improvements cavity-wall insulation
  installed, loft insulation, added double-glazed
  back door and extra double-glazed door to the
  front porch
• play around with thermostat settings - rather
  than fixing the thermostat to a single value, try
  just leaving it at a really low value most of the
  time (say 13 or 15 ?C), and turn it up
  temporarily whenever you feel cold
• current consumption 13 kWh/d!

12
Air conditioning too much Before leaving the
topic of thermostat settings, I should mention
airconditioning. Doesnt it drive you crazy to
go into a building in summer where the thermostat
of the air-conditioning is set to 18 C? These
loony building managers are subjecting everyone
to temperatures that in wintertime they would
complain are too cold! In Japan, the
governments Cool- Biz guidelines recommend
that air-conditioning be set to 28 C (82 F)
this is exactly the 5C we talked about
before.
13
Better Buildings Three no-brainer solutions
(1) have really thick insulation in floors,
walls, and roofs (2) ensure the building is
completely sealed and use active ventilation to
introduce fresh air and remove stale and humid
air, with heat exchangers passively recovering
much of the heat from the removed air (3) design
the building to exploit sunshine as much as
possible.
14
The energy cost of heat How efficiently can heat
be produced? Can we obtain heat on the
cheap? Today, building-heating is primarily
delivered by burning a fossil fuel, natural gas,
in boilers with efficiencies of 7890. Can we
get off fossil fuels at the same time as making
building-heating more efficient? One technology
that is held up as an answer is called combined
heat and power (CHP), or its cousin, micro-
CHP.
15
Much of the heat escapes up the chimney
16
Buildings are the cold place - CHP If there has
to be a cold place, why not use buildings as the
dumping place for this waste heat instead of
cooling towers or sea water? This idea
combined heat and power (CHP) or cogeneration,
and its been widely used in continental Europe
for decades in many cities, a big power
station is integrated with a district heating
system. Proponents suggest micro-CHP, where
tiny power stations should be created within
single buildings or small collections of
buildings, delivering heat and electricity to
those buildings, and exporting some electricity
to the grid.
17
Mythconceptions 1. Often the wrong measure of
efficiency is used, namely one that weights
electricity as having equal value to heat (but
electricity is more valuable than heat) 2.
Second, can waste heat in a traditional power
station could be captured for a useful purpose
without impairing the power stations electricity
production? Unfortunately, no - useful heat to a
customer always reduces the electricity produced
to some degree. The true net gains from combined
heat and power are often much smaller than the
hype would lead you to believe. 3. Contrary to
arguments favoring micro-CHP solutions (for
small buildings), centralized electricity
generation has many benefits in both economic and
energy terms. The benefit for large buildings
is only about 10 or 20.
18
Heat pumps now were talking
Like district heating and combined heat and
power, heat pumps are already widely used in
continental Europe, but strangely rare in
Britain. Heat pumps are back-to-front
refrigerators. Feel the back of your
refrigerator its warm.
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How Does a Refrigerator Work?
The basic heat pump. This same basic structure is
also found in AC units, car radiators, and just
about every other thing that makes coldness. 1)
A (very) cold gas moves through the tubes in the
back of the freezer, absorbing heat. The tubes
absorb heat because, as cold as the freezer is,
the tubes are colder. 2) The (now slightly
warmer) gas runs into the compressor (which
compresses, and is the thing that makes that
humming sound, and in the picture above is
labeled pump). Compressing the gas heats it up.
The gas then passes into the radiator coils
(which radiate, and are found on the back). 3)
Once the gas loses heat to the surrounding air it
drops to near room temperature. 4) The
compressed, room-temperature gas now passes
through an expansion valve (which is a fancy word
for spray nozzle). Expanding causes the gas to
cool (a lot), and it is now ready to absorb heat
from the freezer.
20
Heat Pump Energy Flow
As the illustration shows, an electric heat
pump can deliver more heat to a house than
burning the primary fuel at 100 efficiency
inside the house. That is higher efficiency
than a typical forced-air natural gas furnace
which brings the primary fuel to the house.
This comparison is not quite fair to the
natural gas, however, since you can purchase
natural gas furnaces which deliver the gas energy
into heating at over 90 efficiency.
21
Two configurations A heat pump can cool down the
air in your garden using a heat-exchanger called
an air-source heat pump (figure 21.10).
Alternatively, the pump may cool down the
ground using big loops of underground plumbing
(many tens of meters long), in which case its
called a ground-source heat pump. Heat can also
be pumped from rivers and lakes.
22
Is it geothermal energy? yes and no People
sometimes say that ground-source heat pumps use
geothermal energy, but thats not the right
name (or rather its not the same thing) As we
saw in Chapter 16, geothermal energy offers only
a tiny trickle of power per unit area (about 50
mW/m2), in most parts of the world Heat pumps
have nothing (little) to do with this trickle,
and they can be used both for heating and for
cooling Heat pumps simply use the ground as a
place to suck heat from, or to dump heat into.
When they steadily suck heat, that heat is
actually being replenished by warmth from the sun
(although there is some conduction from below,
the mean ground temperature is controlled by
insolation).
23
Progressive solutions 3 heat pumps In the
future, heat pumps will probably get even
better. In Japan, thanks to strong legislation,
heat pumps are now available with a coefficient
of performance of 4.9 Heat pumps offer a system
that can be better than 100- efficient. For
example the best gas power station, feeding
electricity to heat pumps can deliver a
combination of 30-efficient electricity and 80-
efficient heat, a total efficiency of 110.
No plain CHP system could ever match this
performance.
24

• How do we get 185?
• For example the best gas power station, feeding
  electricity to heat pumps can deliver a
  combination of 30-efficient electricity and 80-
  efficient heat, a total efficiency of 110.
• Example
• Suppose we have a heat pump with a CoP 4, which
  is claimed to be 185 efficient.
• Every 4 W of heating requires 1 W of electricity.
  
• Allow for 8 transmission loss, combine with a
  best gas power production of 53, we get a net
  efficiency of power to the heat pump of (53-8)
  45.
• So 1 W of electrical power from the heat pump is
  1/0.45 2.2 W gas energy.
• The overall efficiency is thus 4 W/ 2.2W 185.

25
Limits to heat pumps As for geothermal energy,
the ground is not a limitless source of heat. The
heat has to come from thermal conduction, which
is slow. If we extract heat too fast one
winter, the ground will not warm enough the
following summer for another season How big a
piece of ground does a ground-source heat pump
need? Assume a community with a high population
density say 6200 people per km2 (160 m2 per
person) Can everyone use ground-source heat
pumps, without using active summer replenishment?
Figure 21.12. How close together
can ground-source heat pumps be packed?
26

• May need summer heating
• A calculation in Chapter E (p303) gives a
  tentative answer of no
• Assuming a heat flow of about 48 kWh/d per
  person, wed end up freezing the ground in the
  winter.
• We must limit extraction rate be less than 12
  kWh/d per person achieved in cities with areas
  of 4 x 160 640 m2 per person.
• Thus we need to replenish heat from summer, using
  either
• heat from air-conditioning, or
• heat from roof-mounted solar water-heating panels
  (this is done by Drake Landing Solar Community in
  Canada www.dlsc.ca), or
• heat from air-source heat pumps too
• The issue of poor winter-time performance of
  air-source pumps, might apply in North America
  and Scandinavia, do not apply in Britain.

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Conclusions CHP will work for large commercial
establishments heat pumps should be widely used
to supply heating and air conditioning for small
businesses and domestic use - these will be
powered by green electricity