 |
Typical horizontal-loop
geothermal installation includes a heat pump in conjunction with
a forced-air system and water heater. Piping loops in the ground
draw latent heat to the house in winter. In summer, the system
carries excess house heat to the ground.
What's
so compelling about geothermal technology?
Energy efficiency.
In a technology defined by numbers, here's a fact that everyone
will understand: With a geothermal system–they're also called
geo-exchange systems–a reasonably tight 2000-sq.-ft. home
can be heated and cooled for about $2 a day.
|
Geothermal Basics
The heart of a typical geothermal system is a ground-source heat pump
that cycles water through an underground piping loop. The water piped
through this loop uses soil temperature to warm or cool the heat
pump's refrigerant. Significantly, the heat pump is located indoors
like a furnace. The geothermal heat pump, with its underground piping
loop, is able to tap a warmer, more stable heat source. The soil
below frost level–4 ft. to 6 ft. deep–stores the sun's
energy at a more or less constant level, with temperatures keyed
to latitude. Subsoil temperatures range from the low 40s in the North
to the low 70s in the South and 55 degrees F soil temperature is
common in much of the Midwest and Central Plains. With a ground temperature
of 55 degrees F, the system needs to boost the heat a mere 15 to
20 degrees to reach a comfortable indoor temperature. The only influence
outside air temperature has on the equation is in the home's ability
to retain heat. Houses lose heat faster on colder days, so all systems
work harder in cold weather. But while a ground-source heat pump
may need to run more often on these days, it doesn't run less efficiently.
The geothermal principle works about as well for air conditioning.
Instead of an outdoor compressor laboring against the heat of the
day having to use hot air as its heat-shedding medium, a ground-source
heat pump operates indoors, using ground temperature as its starting
point. The result is a 20 percent to 40 percent savings over conventional
heat pumps and air conditioners. . On average, however, ground-source
heat pumps deliver three to four times the energy they consume.
An Equipment Overview
While the basic principles of geothermal heating haven't changed much
in 20 years, the technology has, and that has made all the difference.
By far, the biggest step forward has been in new scroll compressors
technology. Scroll compressors are radically different in design
than the old one speed compressors–they use an orbiting coil
instead of a piston–and boast a 30 percent improvement in efficiency.
Because they have very few moving parts–and fewer still that
make contact–they are built to last. Most compressor compartments
also contain two add-ons–a resistance-heat grid and a desuperheater.
Ground-source pumps in northern climates need a little help on very
cold days, and a small electric-resistance heater does the job. The
added operating expense comes to about $30 to $40 a year. Though
this may seem a net loss, it's really not. On-board resistance heat
allows the pump and piping loop to be downsized slightly, which saves
money.
Desuperheater heats most hot water
A desuperheater is an auxiliary heat-recovery system that provides up
to 60 percent of a home's domestic hot water. It's really just a second
condenser located in the cabinet and connected to a standard electric
water heater. It delivers more heat in summer, but it helps in winter,
too. Without a desuperheater, you'd need to install more underground
piping to dissipate the extra heat. As you might expect, most units come
with desuperheaters.
Control Units
Both the thermostats and the control panels for these systems are electronic.
The thermostat is able to sense temperature changes to .1 degrees
F and activate the system when it senses only a 1 degree temperature
drop. Because the human body can sense only a 2 to 3 degree difference
in temperature, the system is always one step ahead in comfort. The
microprocessor in the cabinet does double duty. It sequences the
startup so that less stress is put on equipment, and it also has
a built-in fault sensor that can identify the cause of a malfunction.
The system faults appear on the thermostat so minor problems can
be corrected immediately and more serious problems are diagnosed
before the service technician arrives.
Loop Configurations
In nearly all cases the loop piping is made of flexible, high-density
polyethylene that has a life expectancy of 100 years. Its flexibility
and it's usually 3/4 in. diameter and is joined with heat-sealed
(thermal-fusion) fittings. The most common installation is a horizontal
loop. On average, a horizontal system requires 150 ft. of piping
for every ton of compressor load (12,000 BTUs of heat). A newer 2000
sq.-ft. home will require 2 tons of capacity and roughly 300 ft.
of piping loop. Instead of laying the pipe lengthwise in the bottom
of a long trench, it is coiled in 2-ft. - to 3-ft.-dia. loops like
a large Slinky toy. The coils are laid down and covered with soil.
This "Slinky" method greatly increases surface exposure
and substantially reduces the amount of trenching needed. When a
property won't accommodate this much trenching a vertical, closed-loop
system is the next best option. In this case, a well driller typically
drills holes 150 ft. to 200 ft. deep. The contractor then drops two
pipes joined with a U fitting at the bottom into each hole and joins
all pipes from all holes in a common pit 5 ft. to 6 ft. deep. The
drilling costs more, typically about $1000.00 per hole drilled.
|
