Highly efficient system for producing hot water

Abstract

The present invention relates to a highly efficient system for producing hot water that is comprised of a water-to-water heat pump with optimised subcooling of the coolant, which uses residual heat from low temperature heat effluents, such as grey water, wastewater, industrial effluents, etc. to produce hot water at temperatures of 50°C, 60°C or even higher. The system of the invention achieves high operating efficiency, adapting to the thermal jump of the hot water demand by measuring the temperature difference between the capacitor and a control algorithm, which allows production to be adapted to the requirements of the installation, the heat pump operating at an optimal level.

Description

[0001]

[0002] High-efficiency system for hot water production.

[0003] TECHNICAL SECTOR

[0004] The present invention falls within the field of hot water production and relates to a high-efficiency system for producing hot water from low-temperature heat effluents. To this end, the system of the invention is structured around a water-to-water heat pump with optimized refrigerant subcooling, which uses the residual heat from low-temperature heat effluents (e.g., gray water, wastewater, industrial effluents, etc.) to produce hot water at temperatures of 50 ° C. e C, 60 e C or even higher.

[0005] The present invention achieves high operating efficiency thanks to an innovative heat pump control strategy that adapts to temperature fluctuations in hot water demand by controlling the temperature difference across the condenser. Advantageously, the system of the invention is capable of producing a large volume of hot water from available waste heat with minimal electrical consumption, preferably operating with a natural refrigerant, such as propane, with minimal equivalent CO2 emissions.

[0006] BACKGROUND OF THE INVENTION

[0007] Hot water production systems and processes based on heat exchangers, heat pumps, or a combination of these, are known in the state of the art, in which the residual heat of heat effluents, such as wastewater, grey water, water from different industrial processes, etc., is used.

[0008] Heat pumps typically consist of a condenser, an expansion valve, an evaporator, and a compressor. For a heat pump to operate at maximum efficiency, it must operate under conditions that allow it to be as close as possible to optimal refrigerant subcooling. Theoretically, it can be demonstrated that the thermodynamic optimal subcooling for a given condenser is approximately the subcooling that verifies the condition of having two "pinch points" in the condenser, that is, two points where the temperature difference between the hot and cold fluids circulating through the system is minimal. This condition is complex to determine experimentally, requiring pressure and temperature measurements of the refrigerant, establishing a fixed value for subcooling.

[0009] The article "Closing the Residential Energy Loop: Grey-Water Heat Recovery System for Domestic Hot Water Production Based on Heat Pumps" refers to a system for recovering waste heat from greywater for the production of hot water for domestic use. The system disclosed in this article is composed of a heat recovery exchanger and a water-to-water heat pump comprising a condenser, an evaporator, a compressor, an expansion valve, and a tank between the evaporator and the compressor.

[0010] On the other hand, document EP1724531 A1 refers to a system for heating domestic hot water using the residual heat of wastewater, by means of a heat pump device comprising an evaporator and a submerged pump integrated into a wastewater storage tank, as well as a condenser, a compressor, an expansion valve, a control unit and temperature sensors that may be thermocouples.

[0011] However, these documents do not explicitly detail any specific operating modes for the heat pump that would allow its operation to be optimized under these specific operating conditions for hot water production.

[0012] Knowing the background detailed above, the applicant of the present invention has developed a new high efficiency system for the production of hot water that allows to operate at optimal performance, independently of the water flow and the demand temperature, based on a simple and effective configuration that allows to reduce the cost of systems for the production of water to 60 e C or higher.

[0013] EXPLANATION OF THE INVENTION

[0014] The present invention provides a system for the production of hot water at 60 e C or more with optimal efficiency, based on a configuration and control strategy that makes known systems cheaper and allows operation at or very close to the optimal operating point, without the need to experimentally determine the capacitor's pinch points.

[0015] The system described below is applicable to the production of hot water at temperatures that can range between 50 e C and 90 e C, preferably in facilities where there is some low-temperature residual heat effluent (around room temperature) on site or at a reasonable distance, such as a grey water network in the residential sector or process water in industrial environments.

[0016] Thus, the proposed system offers the advantage of controlling subcooling in a simple and precise manner by directly measuring a parameter, which is the temperature difference between the temperature of the refrigerant in the condenser, T2 hereafter, and the temperature of the water flow to be heated entering the condenser, Ti hereafter, instead of using temperature and pressure measurements in the refrigerant.

[0017] Advantageously, the system of the invention is based on more economical elements than those found in known systems, such as a thermopile formed by a series of thermocouples, a more economical option than a pressure sensor and which directly provides a temperature difference. Additionally, thanks to the efficient control algorithm provided, the proposed system can operate at the optimal operating point, regardless of the flow of water to be heated and its programmed temperature jump.

[0018] The use of a thermopile simplifies the procedure and makes signal processing simpler and more cost-effective, unlike known systems, where the heat pump regulation and control process is not based on measuring temperatures at the condenser inlet, making it necessary to determine the pinch points to achieve operation close to the optimum.

[0019] To this end, the system of the invention comprises a heat pump formed by the following essential elements:

[0020] - a condenser provided with an inlet pipe for water to be heated at a temperature Ti, an outlet pipe for hot water to be supplied to a final consumption point at a temperature T3, a refrigerant inlet pipe and an outlet pipe for refrigerant outlet. The condenser of this heat pump is sized to operate in a wide range of subcooling, preferably between 5 e C and 60 e C, maintaining an approximately constant condensation temperature. Therefore, the condenser is sized for the operating point with the highest subcooling, which is the most critical point for heat transfer;

[0021] - a thermopile composed of two thermocouples in series, a first thermocouple (6) arranged in the condenser's inlet pipe for the water to be heated and a second thermocouple arranged in the condenser's refrigerant outlet pipe; such that the first thermocouple measures the temperature Ti of the water to be heated entering the condenser and the second thermocouple measures the temperature of the refrigerant T2 leaving the condenser, this temperature difference in the condenser being defined as AT C = Ti - T2,

[0022] - a converter connected to the thermocouples of the thermopile that converts the temperature difference AT C measured by thermocouples into a voltage signal,

[0023] - a controller including an algorithm, where the controller receives the voltage signal from the converter,

[0024] - an expansion solenoid valve controlled by the controller,

[0025] - an evaporator provided with a waste heat effluent inlet line, a waste heat effluent outlet line, a refrigerant inlet line and a refrigerant outlet line,

[0026] - a refrigerant accumulator tank arranged at the outlet of the evaporator, and

[0027] - a compressor.

[0028] Thus, the thermopile generates a voltage signal proportional to the temperature difference AT C This voltage signal is received by the converter and the controller algorithm determines the degree of opening of the expansion solenoid valve, the controller actuating the expansion solenoid valve and defining the quantity of refrigerant present in the accumulator tank, that is, the quantity of refrigerant charge in the system, so that AT Cremains constant. Under these conditions, the system's heat pump operates close to its maximum COP, where COP stands for 'Coefficient of Performance', which is the ratio of the power (kW) extracted from the heat pump as cooling or heat, to the power (kW) supplied to the compressor.

[0029] To achieve maximum performance, the refrigerant accumulator located at the outlet of the evaporator is essential, since this component of the system of the invention allows controlling the refrigerant charge in the system, and therefore, maintaining the AT C constant. That is, the configuration described allows maintaining an AT CConstant by varying the subcooling, that is, by varying the effective charge of the refrigerant in the heat pump. This is achieved by acting on the expansion solenoid valve based on the reading made by the thermopile, opening or closing it as necessary to introduce or extract refrigerant from the system, that is, introduce or extract refrigerant into the accumulator tank, which acts as a lung that allows refrigerant to be available or accumulated when necessary to maintain AT. C constant. The specific value of AT C It depends on the type and size of the heat pump used.

[0030] Optionally, between the expansion solenoid valve and the evaporator, the system features a separation tank with a level sensor linked to a bypass valve. The separation tank is connected via a bypass line to the evaporator's refrigerant outlet line. The level sensor determines the opening or closing of the bypass valve, ensuring that the refrigerant enters the evaporator in liquid form, not as a two-phase flow. This results in even distribution of the refrigerant and thus ensures optimal evaporator operation.

[0031] On the other hand, also optionally, although recommended, the system includes a heat exchanger to preheat the network water or effluent to be heated which is at a temperature T oup to the condenser inlet temperature Ti, taking advantage of the heat from the residual heat effluent (grey water, process water, etc.) present in the installation where the system of the invention is installed.

[0032] Thus, the heat exchanger is supplied with the water to be heated to a temperature T o on the one hand, and on the other with the waste heat effluent, such that one of the heat exchanger outlets is connected to the waste heat effluent inlet line to the evaporator; while the other heat exchanger outlet is connected to the condenser's inlet line for the water to be heated.

[0033] Advantageously, the configuration described for the high efficiency system of the invention allows the heat pump to work at the optimal point, that is, thermodynamically at the two 'pinch points', but without the need to determine them by conventional methods, which is achieved based on an easily measurable parameter such as AT. C , controlling the temperature difference at the condenser inlet. The refrigerant used in the heat pump is preferably propane, which contributes to the sustainable nature of the invention.

[0034] Thus, the highly efficient hot water production system recommended offers the following advantages:

[0035] - High heat pump efficiency, with power consumption approximately 20% lower than that of the best known heat pump technology.

[0036] The System is capable of utilizing low temperature waste heat (10 to 25e C for example) to produce hot water at 60 e C or even higher temperatures, with a COP greater than 5.

[0037] - Reduction of global CO2 emissions in hot water production

[0038] - Reduction of residual heat from effluents, such as greywater in residential facilities or process water in industrial facilities, which can be recovered to preheat the water using a heat exchanger.

[0039] - In domestic applications such as hot water production in homes, significant reduction in the gas bill for the boiler with minimum electricity consumption, thanks to the high efficiency of the system, with savings of around €850 per year when compared to a gas boiler for homes with a demand of 100I of water at 60 eC daily. The payback period for replacing gas boilers with the described system ranges from 3 to 5 years.

[0040] - If propane is used as a refrigerant, direct greenhouse gas emissions are negligible compared to the high emissions from current heat pumps using synthetic refrigerants. Propane is not classified as a greenhouse gas, so its use will not be subject to current or future restrictions on the use of such fluids.

[0041] DESCRIPTION OF THE FIGURES

[0042] To complement the description that follows and to facilitate a better understanding of the characteristics of the invention, and in accordance with a preferred example of its practical implementation, two figures are attached as an integral part of said description, representing the following for illustrative, non-limiting purposes:

[0043] Figure 1 shows a diagram of a first preferred embodiment of the system object of the present invention.

[0044] Figure 2 shows a diagram of a second preferred embodiment of the system object of the present invention in which a heat exchanger is included.

[0045] Figure 3 shows a graphical representation of COP versus AT. C for different thermal jumps of the water to be heated, AT a = TI-T3, where data series A corresponds to AT a = 10 e C-60 eC, data series B corresponds to AT a = 30 e C-60 e C and data series C corresponds to AT a = 50 e C-60 e C.

[0046] PREFERRED EMBODIMENT OF THE INVENTION

[0047] Figure 1 shows a preferred embodiment example in which the high-efficiency system for hot water production has a heat pump with the following components:

[0048] - a condenser (5) provided with an inlet pipe for water to be heated (1) to a temperature T1, a hot water outlet pipe (3) to be supplied to a final consumption point at a temperature T3, a refrigerant inlet pipe (4) and a refrigerant outlet pipe (2),

[0049] - a thermopile composed of two thermocouples in series, a first thermocouple (6) arranged in the inlet pipe of the water to be heated (1 ) of the condenser and a second thermocouple (7) arranged in the outlet pipe of the refrigerant (2) of the condenser (1 ); such that the first thermocouple (6) measures the temperature Ti of the water to be heated that enters the condenser (5) and the second thermocouple (7) measures the temperature of the refrigerant T2 that leaves the condenser (5), where AT C = Ti - T2,

[0050] - a converter (8) connected to the thermocouples [(6), (7)] of the thermopile which converts the temperature difference measured by the thermocouples [(6), (7)] into a voltage signal,

[0051] - a controller including an algorithm, where the controller receives the voltage signal from the converter (8), - an expansion solenoid valve (9) controlled by the controller,

[0052] - an evaporator (10) provided with a waste heat effluent inlet line (11), a waste heat effluent outlet line (12), a refrigerant inlet line (13) and a refrigerant outlet line (14),

[0053] - a separation tank (17) with a level sensor (21), such that the separation tank (17) is arranged between the expansion solenoid valve (9) and the evaporator (10), the separation tank (17) being connected by a bypass line (18) provided with a bypass valve (19) to the refrigerant outlet line (14) of the evaporator (10).

[0054] - a refrigerant accumulator tank (15) arranged at the outlet of the evaporator (10)

[0055] - a compressor (16),

[0056] As detailed above, the thermopile generates a voltage signal proportional to the temperature difference AT C, while the converter (8) receives said voltage signal and the controller algorithm determines the degree of opening of the expansion solenoid valve (9), the controller actuating the expansion solenoid valve (9) and defining the amount of refrigerant present in the accumulator tank (15), so that AT C remains constant.

[0057] Thus, the accumulator (15) allows controlling the refrigerant charge in the system, and therefore, maintaining the AT C constant, which allows the heat pump to operate at maximum performance. Thus, when the AT value Cmeasured by the thermopile grows above the entered set point value, the algorithm determines the order that the controller must give to the expansion solenoid valve (9), which will operate in the closing direction so that refrigerant leaves the accumulator (15) located at the outlet of the evaporator (10), which increases the subcooling, and therefore, AT decreases. C . Alternatively, when AT C decreases, the expansion solenoid valve (9) must open to reduce the undercooling and thus AT C increase.

[0058] Figure 3 shows the experimental results obtained for a 50 kW heat pump. As can be seen from the graph, regardless of the thermal jump of the water to be heated AT a = T1-T3, when the temperature difference AT Cbetween the temperature T1 of the water to be heated entering the condenser (5) and the temperature T2 of the refrigerant leaving the condenser (5) is between 3 e C and 7 e C, and more specifically, between 3 e C and 5 e C, the heat pump operates at the optimal COP or very close to it, thus maintaining AT constant C Between these values, the maximum efficiency of the heat pump is achieved without the need to experimentally determine the 'pinch points' using conventional methods.

[0059] In relation to the composition of the thermocouples that make up the thermopile, in an exemplary embodiment one of the thermocouples (6) of the thermopile is formed by an alloy of 90% nickel and 10% chromium while the other thermocouple (7) is formed by an alloy of 95% silicon and 5% aluminum. In an alternative embodiment, one of the thermocouples (6) of the thermopile is formed by an alloy of 55% copper and 45% nickel, while the other thermocouple (7) is made of copper, where both thermocouples are joined by the copper-nickel alloy.

[0060] The tolerances for temperature measurement with the indicated thermocouple compositions would be ±1 K, which, as shown in Figure 3, is perfectly acceptable to ensure that the installation operates close to its optimum. Advantageously, the direct measurement of a temperature difference by the thermopile reduces the measurement error compared to individual temperature measurements with thermocouples.

[0061] Finally, as schematically summarized in Figure 2, in one embodiment of the invention the system has a heat exchanger (20) that is fed with the water to be heated which is at a temperature T0 on the one hand, and on the other with the residual heat effluent at a temperature T res, so that one of the outputs of the heat exchanger (20) is connected to the inlet pipe of the residual heat effluent (11) and the other output of the heat exchanger (20) is connected to the inlet pipe of the water to be heated of the condenser (5), which allows the heat of the residual heat effluent to be used to preheat the water stream to be heated, thus optimizing the energy consumption of the system.

Claims

1 a .- High-efficiency system for the production of hot water, characterized by the fact that it comprises a heat pump consisting of: - a condenser (5) provided with a hot water inlet pipe (1), a hot water outlet pipe (3) to be supplied to a final consumption point, a refrigerant inlet pipe (4) and a refrigerant outlet pipe (2), - a thermopile composed of two thermocouples in series, a first thermocouple (6) arranged in the inlet pipe of the water to be heated (1) of the condenser and a second thermocouple (7) arranged in the outlet pipe of the refrigerant (2) of the condenser (1); such that the first thermocouple (6) measures the temperature Ti of the water to be heated entering the condenser (5) and the second thermocouple (7) measures the temperature of the refrigerant T 2 coming out of the condenser (5), being AT C = Ti - T 2 , - a converter (8) connected to the thermocouples [(6), (7)] of the thermopile which converts the temperature difference measured by the thermocouples [(6), (7)] into a voltage signal, - a controller including an algorithm, where the controller receives the voltage signal from the converter (8), - an expansion solenoid valve (9) controlled by the controller, - an evaporator (10) provided with a waste heat effluent inlet line (11), a waste heat effluent outlet line (12), a refrigerant inlet line (13) and a refrigerant outlet line (14), - a refrigerant accumulator tank (15) arranged at the outlet of the evaporator (10), and - a compressor (16), where the thermopile generates a voltage signal proportional to the temperature difference AT C, while the converter (8) receives said voltage signal and the controller algorithm determines the degree of opening of the expansion solenoid valve (9), the controller actuating the expansion solenoid valve (9) and defining the quantity of refrigerant present in the accumulator tank (15), so that AT C remains constant. 2 a .- High-efficiency system for the production of hot water, according to claim 1 a , characterized by the temperature difference between the temperature Ti of the water to be heated that enters the condenser (5) and the temperature T 2 of the refrigerant leaving the condenser (5) is between 3 e C and 7 e C. 3 a .- High-efficiency system for the production of hot water, according to claim 1 a, characterized in that it has a separation tank (17) with a level sensor (21), such that the separation tank (17) is arranged between the expansion solenoid valve (9) and the evaporator (10), the separation tank (17) being connected by means of a bypass line (18) provided with a bypass valve (19) to the refrigerant outlet line (14) of the evaporator (10). 4 a .- High-efficiency system for the production of hot water, according to claim 1 a , characterized in that it has a heat exchanger (20) that is fed with the water to be heated on one side and with the residual heat effluent on the other, such that one of the outlets of the heat exchanger (20) is connected to the residual heat effluent inlet line (1 1) and the other outlet of the heat exchanger (20) is connected to the water inlet line to be heated of the condenser (5). 5 a.- High-efficiency system for the production of hot water, according to claim 1 a , characterized in that the refrigerant used in the heat pump is propane. 6 a .- High-efficiency system for the production of hot water, according to claim 1 a , characterized in that one of the thermocouples of the thermopile is made of an alloy of 90% nickel and 10% chromium while the other thermocouple is made of an alloy of 95% silicon and 5% aluminum. 7 a .- High-efficiency system for the production of hot water, according to claim 1 a , characterized in that one of the thermocouples in the thermopile is made of an alloy of 55% copper and 45% nickel while the other thermocouple is made of copper.

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