Ventilation Heating With Supercritical CO2 Heat Recovery
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Solution Overview
Problem
Existing ventilation systems in buildings, particularly in passive houses, face challenges in efficiently and cost-effectively heating structures while minimizing energy loss and exergy increase during heat transfer, especially when dealing with varying outside temperatures and the need for heat recovery.
Innovation Solution
A ventilation heater system utilizing an air/air heat exchanger that passes outside air through to absorb heat from exhaust air, with the exhaust air acting as a heat source for a heat pump's evaporator, and CO2 as a refrigerant in a supercritical state, which is cooled and expanded to transfer heat efficiently to supply air, minimizing entropy increase and exergy loss, and includes a refrigerant subcooler to prevent freezing and regulate compressor speed based on demand.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a conventional heat pump system with water-guided heat distribution is used, then heat recovery from exhaust air is achieved, but energy loss and exergy increase during heat transfer occur
Solution Approach 1:
The patent changes the physical state of the refrigerant to supercritical state (above critical temperature and pressure), which fundamentally alters the heat transfer characteristics and reduces exergy loss during the heat recovery process from exhaust air to supply air
Solution Approach 2:
The patent replaces the conventional water-guided heat distribution system with a direct air-to-air heat exchange system using supercritical CO2 as refrigerant, eliminating the intermediate water cycle and reducing thermal losses
2Speed
If rapid heating is achieved through conventional systems, then temperature increase is fast, but sudden temperature jumps and exergy loss occur
Solution Approach 1:
The patent utilizes the unique thermodynamic properties of supercritical CO2, which allows for continuous and smooth temperature adjustment without phase change discontinuities, enabling rapid heating while maintaining thermal stability and minimizing exergy loss
3Temperature
If heat exchanger operates at low outside temperatures, then heating function is maintained, but freezing risk increases
Solution Approach 1:
The patent changes the refrigerant to supercritical CO2, which has a sublimation temperature of -78.5°C, allowing the system to operate reliably at low outside temperatures without freezing risks associated with conventional refrigerants or water-based systems
Solution Approach 2:
The patent converts the potential harmful effect of low temperatures into a benefit by using CO2's sublimation property, where the refrigerant naturally prevents freezing through its thermodynamic behavior in the supercritical state
4Power
If high heating output is provided, then building heating demand is met, but energy consumption increases
Solution Approach 1:
The patent exploits the phase transition of CO2 from supercritical state to gaseous state through throttling, which provides a large heat transfer coefficient and enables high heating output with minimal energy input, achieving efficient heat recovery from exhaust air
Solution Approach 2:
The system uses the waste heat from exhaust air as the primary energy source, with the heat pump system requiring minimal additional energy input to upgrade the temperature level, thereby providing high heating output with low energy consumption
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system achieves efficient heating of buildings with reduced energy loss and exergy increase, allowing for continuous operation without sudden temperature jumps, and can handle low temperatures by preventing freezing and optimizing heating output based on demand, making it suitable for passive houses with low heat requirements.
Implementation Method 1
Outside air is passed through an air/air heat exchanger and absorbs heat from exhaust air in the air/air heat exchanger
Implementation Method 2
exhaust air from the air/air heat exchanger is passed as a heat source to an evaporator of a heat pump, in which a refrigerant is evaporated
Implementation Method 3
The exhaust air exiting the heat exchanger transfers thermal energy to a refrigerant operated in the high pressure area (WPKH) of the heat pump circuit (WPK) in the supercritical state
Implementation Method 4
a compressor for compressing the refrigerant from a low pressure to a high pressure
Implementation Method 5
In the gas cooler of the air heater, CO2 as a refrigerant releases heat to the supply air in a supercritical state
Implementation Method 6
the refrigerant is transferred from a first state with a first temperature (tÜe), which is supercritical, to a second state with a second temperature (tÜa) at an almost isobaric pressure, which is preferably between 73.8 and approx. 80 bar, cooled
Implementation Method 7
In its flow direction, the CO2 is further expanded upstream of the evaporator through a throttle to a pressure below 73 bar
Implementation Method 8
heat is recovered with a heat pump that uses the heat from the exhaust air
Data Source
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AI summary
The method involves heating supply air (ZU) with a temperature (tge) in an air heat exchanger (200) to another temperature (tga) in an air heater (230), where the latter temperature lies above a temperature (tab) of exhaust air (AB). Carbon dioxide is delivered as a refrigerant to the supply air in a gas cooler (170). The refrigerant is cooled at two temperatures (tue, tua) in two states, respectively in the cooler at isobaric pressure between 73.8-90 bar. The carbon dioxide is expanded through a throttle (150) in a flow direction (c) before an evaporator (160) at a pressure of 73 bar. Independent claims are also included for the following: (1) a ventilation heater (2) a building.