Abstract: This paper presents two common methods for utilizing waste heat from gas-driven air-cooled heat pumps during winter heating. It analyzes the characteristics of these two waste heat utilization techniques in terms of structural design, control strategies, equipment installation, and frost formation. Primary energy utilization is used as a key indicator to evaluate the effectiveness of waste heat recovery. The paper provides a qualitative and quantitative comparison of the two methods, aiming to support the design and application of gas engine heat pumps in China.
Keywords: gas turbine; air-cooled heat pump; waste heat utilization; primary energy utilization
Introduction
With the successful implementation of the "West-East Gas Transmission Project," natural gas has become an increasingly important energy source. In the overall energy consumption of society, the construction sector accounts for a significant portion, with refrigeration and air conditioning systems being major contributors. Efficient allocation of natural gas to this sector is essential for optimal energy use.
In the refrigeration and air conditioning industry, natural gas is typically utilized through three main approaches: (1) gas-driven compression heat pump systems, (2) direct-fired absorption heat pump units, and (3) combined or composite systems. Among these, gas-driven compression air-cooled heat pumps are known for their high efficiency, energy savings, low emissions, and flexibility. While widely used in developed countries, such systems are still in the development stage in China. This paper introduces two commonly used waste heat recovery methods from foreign gas engine heat pumps and conducts a comparative analysis to provide insights for domestic applications.
Two Common Waste Heat Utilization Methods in Foreign Gas-Fueled Air-Cooled Heat Pumps
Gas engines typically convert only about 30% of fuel energy into mechanical work, with the remaining 70% released as waste heat. For small to medium-sized gas heat pumps, there are generally two ways to utilize this waste heat: (1) supplying waste heat to the low-pressure side refrigerant, and (2) using waste heat to preheat indoor air.
The first method involves connecting the outdoor and indoor units with two pipes, forming a two-pipe system. Waste heat is recovered from the engine block and exhaust gases and then used to preheat the refrigerant via a plate heat exchanger. This increases the evaporation temperature, improving the system's coefficient of performance.
The second method uses four pipes, allowing waste heat to be directly used for air heating. After the indoor condenser, waste heat is further used to increase the supply air temperature, enhancing the heating capacity of the system. This method has less impact on the heat pump cycle itself but may require additional components.
Other methods of waste heat utilization include direct heat transfer to the compressor discharge, repositioning heat exchangers for greater subcooling, or using waste heat to warm outdoor air. However, these methods are less common due to lower energy efficiency.
Comparison of Two Typical Gas-Fueled Air-Cooled Heat Pumps
The two methods differ in structure, control, and defrost performance. The two-pipe system simplifies indoor unit design, making it ideal for retrofitting existing electric heat pump systems. However, its control system is more complex, requiring careful management of evaporator operation based on outdoor conditions.
In contrast, the four-pipe system allows for simpler control, similar to conventional electric heat pumps. Frost formation is also less severe in the two-pipe system due to the use of waste heat, making it more suitable for colder climates.
To evaluate performance, parameters such as engine efficiency, waste heat recovery rate, and utilization efficiency are defined. Calculations show that under certain conditions, the four-pipe system can achieve higher primary energy efficiency if waste heat losses are minimized.
Conclusion
While the two-pipe system offers advantages in installation and retrofitting, the four-pipe system generally performs better in terms of primary energy utilization when waste heat utilization is efficient. Both systems have their own strengths and are suitable for different applications depending on local climate and system design.
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