Note: Descriptions are shown in the official language in which they were submitted.
Method and system for increasing the coefficient of performance of an air
source heat
pump using energy storage and stochastic control
Technical Field
The present invention relates to a physical arrangement and a control
algorithm that increases
the efficiency of air source heat pump energy collection, and the general
efficiency of heating
and cooling systems.
Background of the Invention
Energy use for the heating and cooling of occupied space, as well as the
provision of heating
and cooling of processes, e.g. cooling of server rooms and provision of
domestic hot water
(DHVV), represent a major portion of global energy use. The United Nations
Environment
Programme estimated global building sector final energy consumption in 2014 to
be
approximately 122 exajoules (EJ), nearly one third of global energy
consumption. About half of
this energy consumption is due to space heating and cooling, and water
heating. Traditionally,
space and water heating have been performed using wood and fossil fuels as an
energy
source, via combustion. In high performance buildings, these roles are
increasingly performed
electrically, particularly through the use of ground- and air-source heat
pumps (GSHPs and
ASHPs). Electricity used is increasingly obtained from renewable energy
sources e.g.
photovoltaics (PV), wind turbines, and biomass, reducing greenhouse gas
emissions
associated with anthropogenic climate change. Also, building-integrated
renewables, e.g. PV
and solar thermal collectors, are becoming more common as local building
energy sources.
While fossil fuels can be stored relatively easily, electrical energy is the
ultimate perishable
good; storage of electrical energy is costly, though slowly declining in cost.
Heat pump (HP) operation efficiency depends on the AT between the temperature
of the
medium from which energy is obtained, e.g. air, soil or water, and the
temperature of the
medium into which the energy is shed, e.g. air or water. Energy obtained from
boreholes (e.g.
60 to 90 m drilling depth) for GSHPs is generally little affected by diurnal
or seasonal climate
variability, resulting e.g. in a COP of between 300 and 500% throughout the
year in temperate
climates, with efficiency of harvest roughly inversely proportional to AT,
though rarely in a
direct, linear fashion. However, the efficiency of energy procurement from the
air for ASHPs is
strongly affected by current ambient conditions. While an ASHP may have a COP
of 500%
under ideal conditions, annual heating efficiency can be reduced to below 180%
in cold
climates. To make matters worse for ASHPs in cold climates, times of highest
energy demand
for space heating due to cold temperatures and/or lack of passive solar gain
from insolation
also generally coincide with lowest COP of the ASHP operation. In extreme
cold, air
temperature may be so low as to prevent ASHP operation altogether, even for
HPs designed
for cold climates, reducing heating to backup electrical resistance use with a
COP of just under
100%.
Summary of the Invention
1. The invention is a combination of a physical system and control strategy
that allows a
significant increase in the efficiency of an energy collector, an air-source
heat pump (ASHP). In
simple terms, it represents a system that allows an ASHP to collect energy at
high efficiency
when the weather is warm and sunny, and to use the energy during times of high
demand
when it is dark and cold, and when the coefficient of performance of the ASHP
would have
been low. One example of an application would be a system which rivals the
efficiency of a
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ground-source heat pump, but at the cost of an air-source heat pump (e.g. at
half the cost of
the GSHP). The invention consists in one part of a physical system with the
ability to store
energy, to disassociate the time of energy collection from the time of energy
use with the intent
of raising the efficiency of energy collection. A water tank is used to store
thermal energy in the
form of hot water. An air-to-water ASHP can collect heat energy and
effectively store large
amounts of heat taking advantage of the relatively high heat capacity of
water. Other options
for thermal storage include cold water for cooling, and phase change
materials, including ice to
make use of both sensible and latent heat storage, and phase change salts or
waxes.
2. Part two of the invention consists of a control system that directs the
ASHP to operate under
conditions conducive to optimizing the devices' COP, to store the energy, and
to direct the use
of the stored energy when required and at times when conditions are
detrimental to achieving a
high COP for the ASHP device.
3. Part three of the invention is the addition of a solar thermal collector,
to dramatically increase
the ability to raise ambient air temperature experienced by the ASHP outdoor
unit in a semi-
enclosed space during favourable conditions, i.e. high incidence of solar
irradiance.
4. Part four of the invention is the addition of a second water tank that pre-
heats water for
domestic hot water (DHW) use in cases where the maximum ASHP output
temperature that
can be produced efficiently is lower than the DHW tank set temperature, to
significantly
increase the amount of energy that can be supplied to create DHW using the
ASHP.
5. Part five of the invention is a simple stochastic (probabilistic) control
algorithm that directs
the HP or device to commence operation from 10 am to 3 pm daily during the
heating season,
when air temperature is highest on average, and when high levels of solar
incidence may
further increase air temperature, and therefore heat pump COP for thermal
energy collection.
6. Part six of the invention consists of an advanced stochastic control
algorithm that uses an
online weather forecast to calculate a building's predicted energy demand,
forecasts the
optimal energy harvest days and times for the duration of the weather
forecast, and directs
each day's energy harvest to collect only enough energy under suboptimal
conditions to supply
the energy required by the building until the optimal collection condition is
encountered. Under
optimal conditions, energy harvest is then continued until either the optimal
collection window
ends, or the energy storage system has been filled.
Brief Description of the Drawings
Fig. 1 - Physical setup of invention components
Fig. 2 - Addition of a solar thermal collector
Fig. 3 - Addition of a secondary tank for DHW heating
Fig. 4 - Advanced stochastic control - control logic
Fig. 2 - Advanced stochastic control - pseudoalgorithm
Detailed Description of the Invention
The present invention consists of a system to increase the efficiency of an
air-source heat
pump (ASHP), by disassociating the time of energy collection and energy
consumption via
energy storage to increase the device's average annual coefficient of
performance (COP), and
by using stochastic (probabilistic) controls to determine optimal energy
harvesting times.
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1.) Part one consists of the physical system that enables efficient energy
collection (Fig. 1). The
system begins with a source of energy, here ambient air (101). The ambient air
is drawn into the
ASHP outdoor unit (102) mounted to the outside of an enclosed, insulated space
(103) The
ASHP outdoor unit extracts energy from the air and the energy is transmitted
via a refrigerant
line (104; return loop omitted) into the insulated, enclosed space (103).
Energy from the
refrigerant is transferred to water inside the ASHP hydrobox (105). The heated
liquid is
immediately transferred using a pump via a pipe (106) into a thermal storage
tank, an insulated
steel tank (Tk-1; 107; return pipe omitted). Energy is removed from the tank
(106) using a
further pump via a pipe (107; return pipe omitted) to its final energy use
(108), space heating via
radiative elements, and production of domestic hot water (DHW). Timing of
operation of the
ASHP outdoor unit (102) is controlled via a control unit (109), described in
part two.
2.) Part two consists of a controller (109) which directs the operation of the
ASHP (106) to
maximize collection efficiency by operating under more optimal conditions for
the maximum
possible amount of time. For heating purposes, this means operation when
ambient air
temperature, and therefore HP COP are relatively high, and storing the energy
to be able to use
when ambient temperatures are low, and when HP COP for operation would have
been low.
The ability of the system to operate under optimal conditions is contingent on
a collector
system (102) whose capacity allows it to collect and store (107) enough energy
during optimal
times to supply energy needs when the ASHP is not operating, and energy demand
exists.)
Control strategies depend on sources of energy, weather patterns and season,
whether heating
or cooling is sought, and many other factors. A thermistor in the storage tank
(107) connected
to the control unit (109) is required to determine storage status. The control
unit (213) also
requires an actuator to activate the ASHP outdoor unit (105) when required.
Two specific
control strategies are described in parts five and six.
3.) Part three consists of a solar thermal collector added to enhance the
ability of the ASHP to
maximize energy collection during optimal conditions. This system consists of
a vertical solar
thermal collector (Fig. 2). In this system, the ambient air (201) is drawn
into a renewable energy
device, here the bottom opening of a vertical solar thermal air collector
(202) attached to the
wall of the enclosed, insulated space. This collector can be constructed from
a clear covering,
e.g. corrugated Poly(methyl methacrylate) [PMMA], a 10 cm air gap, black
corrugated metal
cladding, moisture resistant drywall backing, and painted dimensional lumber
to close off the
sides, at a net zero incremental capital cost, e.g. when compared to costs of
surrounding fibre
cement cladding siding. At times of solar insolation, ambient air (201) is
drawn into the
collector through the stack effect. Incident solar radiation passes through
the PMMA, impacts
on the black metal cladding, is converted to infrared and imparts more energy
to the ambient
air, which then rises (203), enters through a duct (204) and is funnelled into
a semi-ventilated
space, here an unheated, ventilated building attic (205). An air-to-water air-
source heat pump
(ASHP) outdoor unit (206) is located in the attic. The ASHP outdoor unit
extracts energy from
the attic space air pre-heated by the solar collector (202). From here on, the
energy is
transmitted as in part one via a refrigerant line (207; return loop omitted)
into an insulated,
enclosed space, here a utility room (208), energy from the refrigerant is
transferred to water in
the ASHP hydrobox (209), transferred (210) into a thermal storage tank (211))
and finally to its
end use (212). This solar thermal collector considerably increases both the
time periods when
energy can be optimally harvested (cold, sunny time periods, in addition to
warm days), as well
as the average HP COP during the heating season, since under either conditions
ambient air
temperature surrounding the ASHP outdoor unit is increased considerably.
Preliminary testing
shows an increase in air temperature of approximately 20 C resulting from the
solar collector.
4.) Part four consists of a smaller, secondary storage tank (Fig. 3) added
between the energy
storage tank and the domestic hot water (DHW) tank. The production of DHW at
high efficiency
using an air-to-water heat pump is difficult when the desired DHW temperature
is 60 C, while
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the maximum water temperature that the heat pump can produce efficiently is 50
C. By adding
an intermediate tank, the ASHP can raise the incoming city mains water from 4
C to 50 C. This
means that the auxiliary resistance heater in the DHW tank only has to raise
the water
temperature by 10 C to reach the desired 60 C for DHW instead of by 56 C in
the
unaugmented system, reducing resistance heater use by 82%. Apart from energy
flow through
the secondary storage tank (308), energy flow is identical to the base case:
ambient air (301)
enters the ASHP outdoor unit (302), flows (304) to the ASHP hydrobox (305),
then via pipe (306)
to the storage tank (307), and from there into the secondary tank (308) and on
(309) to the
DHW tank (not shown). Energy for space heating (310) still flows directly to
the radiators, and a
control system (311) is still required for efficient HP operation. Combining
the secondary
storage tank (Fig. 3; 308) with the solar thermal collector (Fig. 2; 202)
leads to even greater
overall system efficiency.
5.) Part five is the use of a simple stochastic control system, using a timed
approach based on
empirical data of average air temperature and solar incidence during the
heating season. The
control unit (109) directs the HP outdoor unit (102) to operate from 10 am to
3 pm daily during
the heating season, when air temperature is highest on average, and when
average high levels
of solar incidence further increase air temperature, and therefore heat pump
COP for thermal
energy collection.
6.) Part six of the invention consists of an advanced stochastic control
algorithm (Fig. 4) that
uses an online weather forecast to calculate the building's predicted energy
demand using the
heating degree day (HDD) method, forecasts the optimal energy harvest days for
the duration
of the weather forecast, and directs each day's energy harvest to collect only
enough energy
under suboptimal conditions to supply the energy required by the building
until the optimal
collection conditions are encountered. Under optimal conditions, energy
harvest is then
continued until either the optimal collection window ends, or the energy
storage system has
been filled.
This system extends the simple stochastic control system 5.) by keeping the
same collection
window, but turning the ASHP off under poorer conditions, or limiting
collection to only the
energy required to supply the building's energy needs until the next
collection suboptimum or
optimum is reached.
Verbal description of advanced control algorithm (Fig 4.):
Control algorithm launches at 10 am (401), determines tank water temperature
by averaging
tank thermistor readings (if more than one thermistor present), and
multiplying AT of current
tank average temperature and specified minimum allowed tank temperature with
the tank
water volume and the specific heat of water per C expressed in kWh. Program
obtains three
day online weather forecast and calculates buildings three day energy
requirements using
heating degree day method (see Fig. 5 for details) for space heating and fixed
daily rate for
DHW requirements. Program calculates expected HP COP for each of the three
days (402;
details in Fig. 5). If Day 1 of the forecast is expected to lead to the
highest daily COP of energy
production of the three days modelled (403), run the heat pump until the
maximum specified
tank temperature is reached (404 to 407). If Day 2 is expected to result in
the highest daily COP
(419), run the ASHP just long enough to collect energy requirements to reach
Day 2 collection
cycle (420-424). If day three is predicted to have the highest COP of the
three days (406), there
are two options: if the predicted COP of day two is higher than day one (414),
run the heat
pump on day one just long enough to collect enough energy to meet predicted
energy
requirements until day 2 (415 to 418). If day one has a higher COP than day
two (409), collect
enough energy on day one to supply predicted energy requirements to the begin
of day three
(410 to 413).
The system uses a three day moving time horizon.
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Fig. 5 lists a pseudoalgorithm for ASHP control unit advanced stochastic
controls.
7.) The system can also be used to reduce costs and increase renewable
friendliness by
optimizing for time-of-use or dynamic electricity pricing under a smart grid
tariff structure to
avoid operation under regional electrical grid peak demand times and prices,
through its ability
to operate using stored energy when electrical grid prices are high.
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