Field trials of a waterless home heating and humidification technology.

INTRODUCTIONIt is generally accepted, and has been confirmed by studies, that
humidification of dry indoor air to raise relative humidity (RH) during
the heating season is beneficial to the comfort and health of building
occupants. There are also significant energy savings possible due to the
"apparent temperature" phenomenon that allows people to feel
more comfortable (i.e., warmer) at higher RH. Humidification also
prevents adverse effects on wood floors and furniture and reduces static
electricity buildup which can harm electronic equipment. ASHRAE Standard
62-1989, states, "relative humidity in habitable space preferable
should be maintained between 30% and 60% ... to minimize growth of
allergenic and pathogenic organisms". Notably, the lack of proper
space humidification enhances the rate of influenza virus, resulting in
a significant number of illnesses and deaths each year. Humidity control
is important in commercial buildings -- including hospitals -- as well
as many industrial processes, such as electronic and semiconductor
manufacturing, medical supply, printing application, woodworking and
storage, and textile industries.
Currently, the most widely used residential humidification
technologies are forced air furnace-mounted bypass wetted media, spray
mist, and steam humidifiers. These use city water as a water source and
require additional furnace heat or electricity to evaporate the water.
Mineral deposition, white dust and microbial growth problems are
associated with most of these humidifiers. For commercial building
humidification, demineralized water is typically used for humidification
equipment like steam heat exchangers, electric and ultrasonic
humidifiers, compressed air atomizers, and high pressure cold water
foggers. In addition to the energy consumption for the water
evaporation, energy is also needed to produce high-quality demineralized
water through a reverse osmosis process.
The Transport Membrane Humidifier (TMH) technology was developed byusing a nanoporous membrane that facilitates a capillary condensationseparation mechanism which transports water vapor only from furnacecombustion flue gas to humidify building air. The capillary condensationaction enables high water transport rates while also blockingnon-condensible gases from transporting across the membrane.There are other research efforts aimed at using membranes to
separate and transport water vapor for gas stream dehydration, humidity
control, and energy recovery in commercial HVAC systems. None of these
applications, however, has attempted to extract water vapor from a flue
gas stream to humidify air. For all these and similar applications, only
very small trans-membrane total pressure is available. The driving force
for water vapor to transport from one side of the membrane to the other
side relies mainly on the water vapor partial pressure difference
between the two gas streams. For all these reported applications, they
are dealing with transporting moisture from a high humidity air stream
to a low humidity air stream, the water partial pressure difference is
relatively small, less than 0.4 psi (2,760 pascal).
A flue gas stream typically has a dew point of 120 to 136[degrees]F
(49 to 58[degrees]C). This high temperature high humidity level can
create a greater than 2 psi (13,800 pascal) water vapor partial pressure
difference with the circulating room air, which usually has a dew point
of 50[degrees]F (10[degrees]C) or lower. Using flue gas moisture to
humidify the room air can provide five times larger driving force across
the membrane, therefore substantially less membrane surface area is
needed. The reduced surface area greatly lowers the cost and improves
the prospect for a cost effective commercial application using the TMH.
In addition, since the flue gas is typically at much higher temperature
(over 250UF, or 121[degrees]C), the TMH functions as a heat exchanger to
preheat the air stream to save energy.
The combined energy saving and humidification function with no
potable water consumption makes this technology unique. The reduced
membrane surface area and simple design make it promising for a
commercial product. To our knowledge, no practical technology has ever
been developed for humidifying room air with flue gas moisture for
residential use. TMH technology can reduce fuel use, eliminate city
water consumption, completely avoid mineral deposition and white dust,
and avoid microbial growth, improving both the physical and financial
health of the homeowners.
The TMH technology has been developed from concept to laboratoryprototype, and the laboratory prototype TMH has been tested and provedworking well in a wide operation range for a residential furnace to addmoisture into the circulation air and enhance the mid-efficiencyresidential furnaces (around 80% AFUE) by about 15%. A long term testinghas also been carried out for this laboratory TMH for about 5,000 hourfurnace operating time, which is equivalent to about 4 year operationtime of a typical furnace. At the end of the testing period, the furnaceefficiency still can be enhanced by 13% from its baseline condition,with 5.0 lb/hr (2.27 kg/hr) moisture transport rate to the air, enoughfor home humidification.This paper will mainly introduce two actual home TMH installations
and the test results, to show the TMH technology real world performance
on both whole house humidification effect and furnace efficiency
enhancement in the two occupied homes.
TMH Installation Arrangement And System Setup
As shown in Figure 1 a), the TMH is installed in the furnace air
inlet ductwork. Inside the TMH, flue gas flows from the membrane feed
side, while the room circulating air that requires heating and
humidification flows on the permeate side. The low-temperature,
high-flow-rate room circulating air passing over the membrane surfaces
provides adequate membrane cooling to facilitate the high-performance
capillary condensation water vapor separation mode. Water from the flue
gas is transported to the air side, simultaneously heating and
humidifying the air.
Detailed P&ID of the TMH installations with all the measurement
is shown in Figure 1 b). The furnace natural gas flow rate was measured
by a natural gas flow meter. The furnace flue gas temperature, TMH air
inlet/outlet temperatures, and furnace air delivery temperature were
measured by thermocouples. The air inlet and outlet dew points were
measured by hygrometers. An ID fan was installed to overcome the flue
gas pressure drop through the TMH, and its electrical usage was measured
by a power meter. All experimental data were collected by a data
acquisition system for post-processing.
TMH Module Assembly And Field Installation
Two occupied single family homes were selected to demonstrate the
whole house TMH heat recovery and humidification technology for
residential furnaces, to verify their real world performance on furnace
efficiency improvement and whole house humidification.
Based on the laboratory prototype TMH design and assembling
experience, two TMH modules with even lower air and flue gas pressure
drops were designed, and the two TMH module overall dimensions were
based on the corresponding furnace air ductwork cross sections and their
fuel input capacities. Figure 2 c) shows pictures of the two TMH modules
built for the two field trail installations.
Pictures for the two TMH home installations are shown in Figure 2
a) and b). Furnace in home 1 has a 110,000 BTU/hr (3.22 kW) fuel input,
furnace in home 2 has a 90,000 BTU/hr (2.63 kW) fuel input, both are
mid-efficiency furnaces with AFUE rated 80%. The TMH modules were
installed into the return air ductwork going into the furnaces, and the
flue gas heat and water were simultaneously recovered in the TMH and
distributed into the homes after being further heated by the furnaces.
The flue gas side pressure drops through the TMH were measured as,
0.35-0.4 inches of water (87-99 pascal) for home 1 TMH module, and
0.2-0.25 inches of water (50-62 pascal) for home 2.
Overall Furnace Efficiency Enhancement And Whole House
Humidification Effect
The furnace overall efficiency is calculated based on the fuel
higher heating value (HHV) and the furnace exhaust flue gas temperature
and moisture content. For both mid-efficiency furnaces with the TMH
installations, the flue gas exhaust temperatures decrease significantly
from around 350[degrees]F (177[degrees]C) to around 105[degrees]F
(41[degrees]C) for Home 1, and to around 95[degrees]F(35[degrees]C) for
Home 2. Flue gas outlet dew points decrease from around 125[degrees]F
(52[degrees]C) to around 90[degrees]F(32[degrees]C) for both cases, and
the furnace overall efficiencies thus increase significantly based on
these lower flue gas outlet temperatures and dew points. Calculation
results show that the home 1 furnace efficiency increases from 81.5%
without the TMH to 95.5% with the TMH, and the home 2 furnace efficiency
increases from 80.6% to 96.9%. The average moisture transport rates are
2.7-6.2 gallon per day (10-23 L per day) for home 1, and 1.5-4.8 gallon
per day (5.7-18 L per day) for home 2, depending on different room air
temperatures and dew point conditions. Humidity levels for both homes
have been maintained in a comfortable range of 40-55% relative humidity
with the TMH in operations.
For both homes, we have selected some days to operate the furnaces
under TMH bypass mode to check the baseline conditions without the TMHs.
The results proved a significant humidity increase with the TMH in
operation. For example, relative humidity for Home 1 was 33-38% under
TMH bypass mode, and 40-50% under TMH mode. Figure 3 shows the
humidification effect with and without the TMH operation in January,
2011 for Home 1. This figure shows the temperature and relative humidity
in the first and second floors for Home 1. Figure 4 shows similar
conditions for Home 2, which is a one-story single family home, with
temperature and humidity loggers placed in its family room (FR) and
living room (LR).
Detailed Furnace Performance With The TMH
Different furnaces have different operating characteristics, which
is related to the furnace capacity, the heating area size, and the
customized thermostat programming. The mid-efficiency furnace for Home 1
has shorter heating cycles; and the furnace for Home 2 has longer
heating cycles.
Figures 5 and 6 show the furnace characteristic temperatures, such
as flue gas outlet temperature and dew point, TMH air inlet and outlet
temperatures and dew points, and the furnace final air delivery
temperature, in a typical furnace operation cycle for both homes, under
the TMH mode and TMH bypass mode. From Figure 5, we can see the
circulating air dew point (Td) increases about 3UF(1.7UC) after it
passes through the TMH module in the TMH mode in one heating cycle, but
has no change when the TMH was bypassed. For Home 2 as shown in Figure
6, the heating cycle is much longer, and there is no obvious difference
between the TMH inlet and outlet air dew points, but at the end of the
heating cycle, we can see the air dew point increased about 15UF (8.3UC)
with the TMH, but only increased about 11UF(6.1UC) and stayed at a lower
level when the TMH is bypassed.
Figure 7 shows the instantaneous efficiency of one typical heating
cycle for the two home furnaces at TMH bypass mode and TMH mode.
Averaged efficiency increases for these two typical heating cycles are
from about 82% to 96% for Home 1 furnace and 81% to 96% for Home 2
Economic Analysis and Potential Markets
There are about 35 million gas furnaces currently operating in U.S.
homes. In 1998, 12% of furnaces available in the market are considered
high efficiency furnaces, and by 2010 high efficiency furnaces
represented about 30% of national furnace shipments. So it is estimated
now more than 70% of furnaces in use are still mid efficiency furnaces
(Federal requirement for minimum 78% AFUE, most of them are around 80%
AFUE). This TMH technology is first targeted for this huge retrofit
market to significantly boost the mid-efficiency furnace efficiency
while at the same time providing whole house humidification without
external water consumption and other health benefits. The high
efficiency furnace shipment percentage is not expected to increase
significantly in the near future considering various federal and state
high efficiency rebates are winding down, and the payback period is less
attractive to customers for the much higher equipment cost of the high
efficiency furnaces, which are typically more than doubled of the mid
efficiency furnace price. Although have not been demonstrated yet, the
TMH technology has already been further developed and proved in our
laboratory to have the potential to be used for high efficiency furnaces
too. Many of the high efficiency furnaces have lower than 92% AFUE, only
a small amount of flue gas water vapor in these furnaces are condensed
therefore the remaining water vapor is still enough to humidify the
homes, though the efficiency gain by the TMH installation will be lower
for these furnaces. For much higher efficiency furnaces, the TMH modules
can be built into the furnaces to replace their condensing heat transfer
modules, so all the flue gas water vapor is available for the home
humidification. Table 1 summarizes how the TMH stacks up against main
conventional humidifier types. Besides the energy and health benefits
listed in the table, there is no wetting medium needed to be replaced
regularly during a heating season compared with conventional
humidifiers, which typically costs about $30/year. The payback period of
the TMH installation for a mid efficiency furnace is estimated at less
than 4 years.

Table 1. Comparison of Current Furnace Humidifier Types with
Proposed TMH

Commercial Proposed

Type Bypass Steam Spray mist TMH
humidifier humidifier humidifier

Additional 4% 0 4% 0
furnace fuel

Electricity 12 watts 1,400 watts Negligible 20 watts

Mineral Yes Yes Yes (a) No

"White dust" Medium Zero High Zero
in home

Microbial High None Very low Very low

Water 15 gal/day 15 gal/day 12 gal/day Zero

Equipment $150-$225 $525-$850 $160-$200 $400 (c)

(a.) potential clogging of spray nozzle; also requires water filter.

(b.) assumes average 3 gal/day additional water throughput to
control mineral deposits.

(c.) preliminary cost target.
Two field trail TMH units were designed and tested for two typical
mid-efficiency residential furnaces in two occupied single family homes.
The real world operating results showed the TMH units are capable of
transferring enough water vapor from furnace flue gas to circulating
room air for humidification, and enhancing furnace efficiency by about
15%. The home room temperature and humidity continuos monitoring data
indicates both homes have been humidified to a comfortable humidity
level (40 to 60% RH) with the benefits of no external water consumption,
no white dust and no baterial growth concerns. For the two heating
season operation of the two TMH units, the technology was proved can
provide comfortable and healthy humidification for the home owners and
also greatly increase their furnace efficiencies. The TMH technology
will be first targeted for the existing furnace retrofit market, and
further development is for emerging high efficiency furnace market for
both retrofit and new installations.
This work was sponsored by the Utilization Technology Development
RH: relative humidity
HHV: Higher Heating Value
[T.sub.d]: dew point
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Dexin Wang, PhD
Shawn Scott
Ainan Bao, PhD
William Liss
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