Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A PROCESS AND APPARATUS FOR RECOVERING ENERGY FROM
WASTEWATER
Field of Invention
The present invention relates to a process and apparatus for the removal of
residual
heat from wastewater for reuse. The preferred embodiment described as an
example
herein is particularly suited to the removal of heat from grey water in
domestic
residences, for later reuse in supplying energy for water heating in the home.
Background
Globally, two key factors reducing the environmental sustainability of housing
are
water consumption and energy use. Consumption of these resources is increasing
due
to increasing population. Pollution is reducing the availability of fresh
water, while
efforts to reduce pollution increase energy production costs. The problem is
compounded by localized population growth in greenfield developments in
sunbelt
areas (where water resources and infrastructure are scarce) and by the effects
of a
changing climate. With the energy embedded in potable water, through
purification
and distribution, accounting for a significant proportion of global energy
generation ¨
the 'water-energy nexus' ¨ opportunity exists to target both problems in an
integrated
way.
Wastewater discharged from domestic premises is a notable loss of energy from
the
typical home. Reduced energy consumption through the recovery or elimination
of
waste heat has seen increasing adoption, through heat recovery ventilation
(HRV)
systems, low-e glass and improved insulation; yet wastewater heat has received
little
attention. It is a challenge for two reasons: the contaminant load will
quickly block
efficient, single pass heat exchangers, and the irregular pattern of waste
production
limits the amount of heat that can be easily recovered. Incumbent solutions
simplify
the process by targeting relatively clean shower water (where warm waste and a
requirement for delivery of hot water coincide), but this does not tap the
full potential
for energy savings.
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In situations where wastewater treatment is also desirable , the removal of
heat from
the wastewater stream presents additional challenges for the treatment
methodology.
Biological systems, for example, rely on the action of microorganisms to
digest
contaminants, but these processes are slowed as the temperature is reduced.
Similarly,
low temperatures reduce the efficiencies of many filtration and chemicals
systems.
Some flocculation processes, for example, fail completely outside an optimal
temperature range.
Newly developed wastewater treatment processes (such as PCT2010902814)
eliminate this temperature dependence, enabling the integrated approach
described by
this invention.
Existing methods of high efficiency water heating technologies include solar
collectors and air-sourced heat pump systems, but these technologies suffer
major
flaws that have prevented their widespread adoption. Solar applications
operate only
during daylight hours and are dependent on weather (producing reduced output
during
cloudy weather). While evacuated tube solar collectors are largely independent
of
outside temperature, they are vulnerable to hail. Alternatives can be subject
to
bursting during freezing temperatures. Air-sourced heat pumps present an
alternative,
but their efficiency is highly dependent on outside air temperature, making
them
unsuitable in cold climates. Passive systems are also available and, while
cost
effective, they do not preform reliably, place waste and potable streams in
close
proximity, and do not tolerate highly contaminated waste streams.
There is, therefore, a need in many circumstances for a more reliable, robust
form of
heat capture device that can supply the domestic hot water needs in an
efficient way.
It must also be compatible with wastewater treatment technology to ensure that
direct
energy consumption for water heating, and the flow-on effects of supplying
water,
can be addressed.
Summary of the Invention
In one respect, the current invention resides in a process for the removal of
heat
energy from wastewater that comprises:
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a) Collection of wastewater in a reservoir;
b) Transfer of heat energy from the collected wastewater into a working
fluid;
c) Transport of the working fluid;
d) Transfer of the heat energy from the working fluid to a heat energy
reservoir; and
e) Transfer of the heat energy, on an as-needed basis, from the heat energy
reservoir to the incoming potable water supply.
According to the embodiment described, the preferred method of heat transfer
is using
a heat pump, where the working fluid is a refrigerant gas. Many commercially
available refrigerant gases are suitable, including R-134a, R-417a, R-744, R-
600a, R-
410a; with R-417a being the most preferred according to this invention.
Preferably, the process further comprises:
a) Passing the influent wastewater over a filter to remove particles larger
than
approximately 200 m;
b) Delivery of the wastewater, with heat energy removed, to a wastewater
treatment system for water recovery.
In another respect, the invention resides in an apparatus for the removal of
heat
energy from wastewater. The apparatus comprises:
a) A device allowing seamless interconnection between the wastewater supply
and the process described herein;
b) A heat exchange surface in contact with the wastewater;
c) A heat storage reservoir containing heat storage media;
d) Heat exchange surfaces in contact with the heat storage media;
e) Plumbing to transfer working fluid between the heat exchange surfaces;
In the preferred embodiment, the apparatus further comprises:
a) A compressor to move the working fluid between the heat exchange
surfaces;
b) A thermostatic expansion valve, or a capillary tube, to promote a phase
change in the working fluid;
c) Temperature measuring devices;
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d) A thermostatic control system;
e) A supplemental, backup heating device;
f) Phase change media in the heat storage reservoir; and
g) Plumbing connecting the potable water supply to the heat storage device;
Additionally, the interconnection device may further comprise one or more of
the
following:
a) A detention chamber to buffer and smooth peak loads on the system;
b) A wastewater backflow prevention device;
c) A pump for the transfer of cooled wastewater to a treatment system;
d) A level sensor to control the pump;
e) An integrated filter capable of removing particles larger than
approximately
200 m;
f) A sealed lid to prevent the ingress of water;
g) A vent to encourage reliable flow of water;
The process according to this invention is ideally suited to the treatment of
domestic
grey water. 'Grey water' is wastewater produced from fixtures including
showers,
hand sinks and laundry facilities. These fixtures are not designed for the
collection of
human excrement or discharges, and faeces or urine does not grossly
contaminate the
resulting wastewater.
Grey water fixtures generally include major sources of hot water consumption,
and
have warmer resulting waste streams. It is important that these are captured
to ensure
the invention described herein operates at the highest efficiency.
Description of Drawings
The following drawings describe, in a non-limiting way, the invention with
respect to
a preferred embodiment:
Fig 1. Describes the process of the invention and how it may be installed to
capture heat energy from domestic grey water.
Fig 2. Presents a detailed cross-section of an appropriate interconnection
device.
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Detailed Description of the Invention
The invention resides in a process for the recovery of heat energy from waste
water.
The invention is suitable to application to many types of wastewater;
preferably this
water is generated in domestic residences. The invention can also be applied
to other
waste streams where warm water is generated, and a need to heat incoming water
coincide. Water processed according to this invention must be above 0
Celsius, and
preferably wastewater should be generated between 15 and 65 degrees.
According to Figure 1, a preferred embodiment is depicted in which the
invention is
coupled to the grey water sources (100) in a domestic residence (101).
Wastewater is
directed, via separated plumbing (102) designed to maintain separation between
grey
water and more heavily contaminated streams, to an interconnection device
(103).
The interconnection device has a number of functions, including acting as a
buffer to
decouple the generation of wastewater from the treatment, heat capture and
reuse
processes. Each of these processes operates according to a differing (regular
or
irregular) schedule, and the decoupling effect allows them to be
interconnected. Inside
the interconnection device, a heat exchange surface (104) is placed in contact
with the
collected wastewater. The device additionally comprises connection to the
conventional sewer system (105) or an alternative method of wastewater
disposal
such as a leach field, septic system, holding pond or aerated wastewater
treatment
system. A separate connection to a suitable grey water treatment system (106)
is also
included. This connection might alternatively be used to supply grey water for
applications other than treatment and reuse.
The heat exchange surfaces act as a barrier between the grey water, contained
in the
interconnection device, and a refrigerant working fluid contained by a network
of
refrigerant plumbing arranged in a closed loop (107). The plumbing can be made
from a great number of materials, with copper being the most preferred. The
loop
operates in such a way that refrigerant is expanded from a liquid phase, by
means of
an expansion valve (108) or a capillary tube. The cold gas is passed through
the heat
exchange surface where its temperature is raised by energy transfer from the
collected
wastewater. This warm gas can then be compressed using the compressor (108),
where it is passed to another heat exchange surface (109). This heat exchange
surface
is in contact with heat storage media (110) in a heat storage reservoir (111).
The hot
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refrigerant is able to transfer energy, via the heat exchange surface, into
the media.
The remaining plumbing (112) then returns the refrigerant fluid to the
expansion
valve and the cycle is repeated.
If a second heat exchange surface (113) is placed in contact with the heat
storage
media, energy can be transferred to cold incoming potable water (114). This
system
operates most efficiently when there is a high difference in temperature
between the
storage media and the incoming water supply. For this reason, the heat storage
reservoir will be insulated and will preferably contain some form of
supplementary
heat source (115). This heat source may be of any available type, including:
electric
element, gas fire, solar, geothermal, wood fire, among others as appropriate.
If the
storage media is water, it is desirable that the temperature be maintained
above 65
Celsius in order to prevent the growth of thermophillic bacteria. The water
must also
be prevented from boiling, by remaining below 100 Celsius, to prevent undue
pressure on the structure of the reservoir. A reservoir containing water would
require
a pressure relief valve. Alternatively, the heat storage reservoir may contain
a phase
change material, such as paraffin wax, capable of storing larger amounts of
heat
energy for a given volume.
In some circumstances, it is desirable that the heat energy storage media
(110) is itself
the potable water supply. In this instance, the heat exchange surface (113) is
replaced
by an open pipe allowing direct flow of potable water into, and out of, the
reservoir.
For applications of this type, a tempering valve (116) is placed between the
heat
energy reservoir and any residential fixtures. This ensures water supplied to
the home
is maintained at a temperature suitable for application to human skin.
The invention further resides in an apparatus for the recovery of energy from
wastewater. The apparatus facilitates the extraction of heat energy from water
streams
by providing an interconnection between heat transfer equipment and the waste
that
would be sent directly to sewer in ordinary circumstances. The apparatus also
provides a connection between incoming, cold, potable water and heat stored in
an
energy storage reservoir, along with a mechanism to transport heat energy
between
the interconnection device and the reservoir. The heat transfer mechanism
might
comprise a direct heat transfer, dilution of one stream with another,
conversion to
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mechanical or chemical energy, or conversion to electrical energy. In the
preferred
embodiment, energy is transported by means of a refrigeration cycle, or heat
pump.
According to Figure 2, the interconnection device comprises a waste inlet
(200),
where wastewater, preferably including hot domestic grey water, enters the
device.
The interconnection device may be made of a wide range of suitable waterproof
materials including plastic, metal, composites, or natural substances.
Preferably, the
material should be smooth, to reduce the adherence of wastewater contaminants
and
should have low thermal conductivity, to reduce the loss of heat energy to the
surrounding environment. In many applications, the plumbing connection will be
below the surface of the ground, so the design of the device must be such that
it can
withstand ground pressures. It must also have features that prevent it being
lifted from
the soil due to uplift pressures, frost, or a water table that rises
periodically. The
preferred embodiment described here would be constructed from an insulating
plastic
material (201) of sufficient thickness to attenuate heat losses to the
surrounding soil.
Water entering via the inlet is then optionally passed across a filter (202)
where
particles larger than a prescribed size are separated from the main flow. The
filter may
be of many configurations, and may be washable, disposable or self-cleaning.
Preferably, a self cleaning filter incorporating wedge wires designed to
separate
particles larger than approximately 200 m is used. Collected particles (203)
are then
directed, using a small amount of incoming wastewater, toward the outlet
(204).
Water, with larger particles removed, is then directed into a heat extraction
chamber
(205), where it is placed in contact with a heat exchange surface (206). The
heat
exchange surface is also in contact with a refrigerant working fluid in the
preferred
embodiment. The working fluid is connected to the rest of the process by means
of
plumbing with an inlet (207) and outlet (208). For best efficiency, it is
preferable that
the inlet is nearer the top of the chamber, and the hottest water, and the
outlet nearest
the bottom of the chamber.
Alternatively, the heat exchange surface could be directly in contact with the
potable
supply, or could comprise an alternative method of energy transfer, like a
Peltier
device. The heat transfer chamber is designed such that newly incoming water
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contacts the heat exchange surface before being directed into the main body of
the
tank. This ensures that the temperature of water in contact with the heat
exchange
surface is at the highest possible temperature, which promotes efficient
transfer into
the working fluid, and thus efficient operation of the apparatus.
Domestic wastewater is generated in a pulsatile fashion, for example the
emptying of
a bath. For this reason, the heat extraction chamber is sized such that it can
contain
pulses of wastewater for a sufficient period to extract the maximum amount of
heat.
Preferably, in a typical home, the size of the chamber is between 50 and 500
litres,
with between 100 and 150 litres being most preferred. Other applications will
require
the chamber to be sized differently as appropriate.
Temperature in the heat extraction chamber is maintained above a critical
point
through the use of a thermostatic control system (209) and a temperature-
measuring
device (210). In the case of water, the temperature must be maintained above
the
freezing point, 0 Celsius, to ensure that ice does not form. Ice reduces the
efficiency
of heat transfer by insulating the heat exchange surface from any incoming,
warmer
water. It can also block flows and disrupt the intended operation of the
interconnection device. The low temperature threshold for the thermostatic
control
system is set between 00 and 4 Celsius, with between 2 and 04 degrees being
the
optimum range according to this invention. At temperatures below this
threshold, the
heat energy recovery process is stopped until further wastewater is gathered
and the
temperature returns above the threshold. To reduce starting and stopping
frequency, it
is desirable that the thermostatic control system includes a lag, for example:
turning
off the heat recovery equipment when the temperature falls to 2 Celsius, and
activating it again only when the temperature rises to 4 C.
Water in the chamber can be directed through the use of baffles (211) and
weirs (212)
to ensure that the hottest water remains in contact with the heat exchange
surface for
the maximum time possible. Optionally, it can be directed by mechanical means,
such
as a recirculation pump or mechanical mixer arranged in such a way as to
maintain
turbulent flow and circulation within the chamber.
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After the heat has been recovered, water is directed to a second storage
chamber by
passing over a final weir (212). This chamber (213) is designed to uncouple
the
pulsatile output of water generated by the residence, from the processing
requirements
of a downstream wastewater treatment system. In may cases, these systems
operate in
a continuous fashion, or have a defined batch size that would be otherwise
incompatible with the output from a typical home. The size of the chamber will
depend, in particular, on the specific treatment process being used, but will
typically
be between 50 litres and 2000 litres. Most preferably, the detention chamber
will have
a capacity between 100 litres and 300 litres. Water can be directed from the
detention
chamber to a suitable wastewater treatment system by means of a pump (214),
optionally controlled by a level sensor (215), and interconnecting plumbing
(216).
In some cases, water may not be transferred from the detention chamber at the
same
rate it is being generated by the residence. In these cases, the detention
chamber may
become full and water will exit the chamber via an overflow port (217),
traveling
directly to the conventional sewer system, or other conventional method of
wastewater disposal, via a plumbing connection (218). In these circumstances,
the
outgoing wastewater will carry away any remaining particulate matter that has
been
collected by the filter.
Finally, the interconnection device is designed to prevent any discharge of
malodorous gases to the local environment. A lid (219) with a tight seal (220)
also
prevents the ingress of ground water, or rainwater. Some ventilation is
required to
ensure the reliable flow of liquid through the device, so a connection to a
standard
plumbing vent is required. The device may also include a backflow prevention
system
(221), designed to prevent waste from the conventional sewer system from
entering
the system by the reverse path and contaminating the grey water with heavily
soiled
streams.
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