Using excess summer
heat from solar collectors to drive desiccant cooling systems is often
proposed. A two wheel desiccant system using solar heat for desiccant
regeneration is typically discussed. The two wheel system uses a desiccant
wheel that is “matched” with a heat exchanger wheel. The heat exchanger recycles
heat for the desiccant regeneration and improves system efficiency. These
systems are generally limited to delivering warm dry air or cool humid air in
most parts of the US.
A newly patented
desiccant cooling cycle creates two dry air streams. This new cycle uses
indirect evaporative cooling of one air stream to cool the second stream.
Additional direct evaporative cooling allows cool and dry air to be delivered
to the building. Regeneration exhaust heat can provide water heating. Combining
the system with a new solar air heating system should provide a significant
solar heating, cooling, and hot water delivery system.
Low temperature
heating dominates all residential, commercial, and industrial end uses of
energy within buildings. Nearly 61% of the energy used across all sectors of
the economy is for low temperature heating uses. These heating end uses include
space heating, industrial process heating, water heating, boiler heating, and
clothes drying. The second greatest
energy end use is for cooling. Another 13% of all building energy end use is
for refrigeration or space cooling. In the late 1990's, the combination of all
heating and cooling energy end uses cost US consumers nearly $180 billion per
year.
Most of this energy
use requires the consumption of fossil fuels. In most cases, the energy
conversion devices, such as boilers or electric heat pumps, operate at low
efficiency compared to the fuel they consume. In almost all fossil energy
heating and cooling systems, the conversion from the primary fuel (gas, oil,
coal, etc.) to heating or cooling is done at less than 100% efficiency. This is
often described as a coefficient of performance of less than 1.0. (COP < 1).However, there are
three technologies that operate at what can be called super efficiencies. These
technologies convert primary energy into heating or cooling capacity with COP’s
between 2 and 5.
These three
technologies include solar thermal heating, evaporative cooling, and desiccant
drying. By using a combination of simple, low energy, physical phenomena, and
widely distributed low cost energy and water resources, these three
technologies are recognized as super efficient at delivering heating and
cooling.
Solar air and water heating system have been
shown to have a COP of 4 or greater at providing solar heat with little expense
in fan or pumping primary energy. Evaporative cooling systems have been
demonstrated to operate at a COP of 5 when used in dry conditions. Desiccant
system with evaporative final cooling can operate at a COP of more than 2.
However, the
application of these technologies has been restricted by their individual
limits in responding to:
1) High temperature or
2) High levels of humidity, or
3) Harsh economic realities caused by seasonal
idleness of expensive heating and cooling equipment.
Evaporative cooling
is only effective for comfortable cooling in dry climates. When outdoor
humidity rises, the cooling capability of direct evaporative systems declines
unless occupants are willing to suffer with high humidity. Even in climates
that suffer only a few weeks of high humidity, most consumers will select low
efficiency compression refrigeration cooling systems for comfort cooling. Since
most customers buy only one system, the low COP compression systems will be the
only cooling systems installed. Thus, the rest of the year’s super efficient
evaporative cooling capacity is lost for lack of a few weeks of dry air.
Solar space heating suffers a similar fate due
to the typical high cost of flat plate collector systems and the lack of useful
energy cost savings delivered in the summer months. The seasonal decline in
cost savings reduces the overall cost savings the systems can deliver in any
given year. This stretches out payback periods for traditional flat plate systems
and makes them uneconomic for most space heating applications.
Additional research
is recommended in the following areas:
1) Evaluate
alternative solar tile roof components to establish the most cost effective
summer outlet temperature for the solar roof when supporting desiccant
regeneration.
2) Prepare a
desiccant wheel that is optimized to the outlet temperatures of the solar roof.
3) Assemble the
components in an operational prototype and test for cooling performance in a
suburban setting.
0 comments: