Heat pump system design considerations
For the economic operation of a heat pump system, there are several basic requirements that should be met:
1. It is essential to utilize high-temperature heat sources as much as possible. This helps improve efficiency and reduce energy consumption.
2. The actual temperature difference between the heat source and the heat medium should be kept as low as possible. A smaller temperature difference ensures better heat transfer efficiency and reduces the workload on the system.
3. The heat source must meet the following criteria:
- It should provide the required heat at the highest possible temperature to maximize system performance.
- The cost associated with using the heat source should be as low as possible to ensure economic viability.
- It should not have any harmful effects on the heat exchange equipment, whether chemical or physical in nature.
- It should be adaptable to changes in heat conditions, ensuring stable operation under varying environmental circumstances.
- Transmission equipment such as fans and pumps should consume minimal energy to maintain overall system efficiency.
4. The compressor in the heat pump system should meet the following requirements:
- For air-source heat pumps, the compressor should operate efficiently within a temperature range of -35°C to +15°C, with a maximum discharge temperature of 65°C. The compression ratio should be as high as 9, while maintaining good efficiency. Additionally, the superheating temperature should not be excessively high.
- The compressor should be able to handle superheated and unsaturated steam without being affected by the quality of the suction gas.
- It should be capable of withstanding rapid pressure fluctuations without causing oil foaming. The lubricating oil should return to the crankcase with the refrigerant vapor and be maintained with immersion heaters.
- The system should include reliable over-temperature and over-voltage protection devices, ensuring trouble-free operation for more than 25,000 hours.
- Larger heat pump systems should use a smooth and lossless working medium to optimize performance and reduce energy losses.
- The motor accompanying the compressor should have a flat efficiency curve, ensuring consistent performance across different operating conditions.
Based on these requirements, the following recommendations are proposed:
- For heating capacities ranging from 0.5 kW to 500 kW, positive displacement compressors are recommended.
- For heating capacities above 500 kW, centrifugal compressors are more suitable due to their higher efficiency and scalability.
5. Refrigerant Flow Control
To ensure proper operation, thermal expansion valves should be used, preferably of the "balanced hole" type. This design helps maintain stable pressure despite changes in operating conditions, while keeping pressure drops to a minimum for efficient system performance.
6. Economic Evaluation
A heat pump system is generally considered economically viable if the following conditions are met:
- There is a reliable and high-quality heat source, such as groundwater.
- The annual operating time is at least 2000 hours, ensuring a reasonable return on investment.
- The coefficient of performance (COP) should be greater than 3, indicating efficient operation.
- In summer, the system can be used for cooling while still maintaining an efficient heating cycle without requiring additional energy input.
- The system can be integrated into existing refrigeration applications, such as cold storage facilities, skating rinks, and supermarkets.
- There is available waste heat that can be utilized, especially in large buildings where internal waste heat can be used for space heating when ambient temperatures drop below freezing, such as in department stores, theaters, and computer rooms.
Conversely, a heat pump system may not be economically viable in the following cases:
- If it is only used for heating and not combined with other functions like cooling.
- If the COP is less than 3, indicating poor efficiency.
- If the heat pump capacity is significantly larger than the actual heating load, leading to unnecessary energy consumption and inefficiency.
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