CA3008508A1

CA3008508A1 - Method and system for increasing the coefficient of performance of an air source heat pump using energy storage and stochastic control

Info

Publication number: CA3008508A1
Application number: CA3008508A
Authority: CA
Inventors: Godo A. Stoyke
Original assignee: Individual
Current assignee: Individual
Priority date: 2018-06-15
Filing date: 2018-06-15
Publication date: 2019-12-15

Abstract

A method and system for increasing the annual average coefficient of performance (COP) of an air-to-water air-source heat pump (ASHP) through the use of thermal storage and a control system that activates ASHP operation during times favourable to more efficient operation. This method uses thermal storage to disaggregate the time of energy production and the time of energy use to increase the COP of the heating system. The efficiency of the system can be further increased through the use of a solar thermal air collector, and a secondary thermal storage pre-tank for domestic hot water (DHW). A simple stochastic control uses a timed approach to maximize ASHP operation during hours during the heating season that coincide with empirical historic maxima in average air temperature and maximum solar irradiance. An advanced stochastic control algorithm extends the simple stochastic model to use online weather forecasts to calculate projected daily building energy requirements based on information of the building's modelled annual energy requirements and determines optimal and suboptimal harvest days. The control algorithm has the ability to maximize ASHP operation on days that will yield the highest annual COR In modelling, the system described has the ability to double the annual efficiency of the ASHP for space heating and DHW, cutting annual energy costs and greenhouse gas emissions in half. The system can further reduce costs and increase renewable friendliness by optimizing for time-of-use or dynamic electricity pricing under a smart grid tariff structure.

Description

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 1 of 5 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.
2 of 5 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 3 of 5 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.
4 of 5 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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Claims

1. A method of significantly increasing the average annual coefficient of performance (COP) of an air-to-water air-source heat pump (ASHP) for space heating and domestic hot water (DHW) by disassociating time and date of energy collection from time and date of energy use through thermal storage, and employing a control system that runs the heat pump during conditions that favour maximal heat pump COP rather than during times of maximal energy demand. This method can significantly reduce energy consumption and costs.

2. The method of claim 1 wherein the ability to increase COP is increased through use of a solar thermal collector of vertical, angled or horizontal orientation, and based on collection of hot air or hot water.

3. The method of claim 1 wherein the the ability to increase COP of DHW
production is increased through use of a secondary pre-DHW tank that allows pre-heating of water for DHW use.

4. The method of claim 1 wherein the control algorithm consists of a simple stochastic model using a timed approach for operation.

5. The method of claim 1 wherein the control algorithm consists of an advanced stochastic model using online weather forecasts to determine expected building energy requirements and expected daily ASHP COPs to determine optimal energy collection strategies for the duration of the forecasts, determining the current days ASHP operation.

6. The method of claim 1 wherein the system is designed for cooling rather than heating. This can include the use of cold storage, and optimization for cooling COP.

7. The method of claim 1 wherein phase change (latent heat) is used in addition to sensible heat. Phase change materials can include phase change salts and waxes. Thermal storage materials can also include water and admixtures of water and propylene glycol or other liquids.

8. The method of claim 2 wherein the movement of air or water through the collector is enhanced by use of a fan or a pump, respectively.

9. The method of claim 1 wherein the ASHP is replaced by another energy collector, e.g. a simple counterflow heat exchanger.

10. The method of claim 1 wherein the system is also optimized for the avoidance of higher energy costs where peak demand charges, time-of-use charges, day-time and night-time tariffs or a smart grid provide opportunities to optimize the system for a blend of cost reduction through energy efficiency and energy consumption, and cost reduction through avoidance of peak penalties, or to avoid carbon taxes, or to minimize greenhouse gas emissions.

11. The method of claim 1 wherein the system is used for industrial and other processes rather than space conditioning and DHW, including process heating and cooling.