What? Natural gas can actually be used for cooling after burning!

Produced by: Science Popularization China

Author: Tian Jiayao (Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences)

Producer: China Science Expo

Generally speaking, after natural gas is burned, it can provide a large amount of heat energy for people to keep warm and cook, but did you know that burning natural gas can also be used for cooling!

Figure 1 Domestic pipeline natural gas

(Photo source: Veer Gallery)

What is a gas heat pump

As we all know, ordinary water pumps can pump water from a low level to a high level, so what is a "heat pump"?

The second law of thermodynamics tells us that heat can be transferred spontaneously from a high-temperature object to a low-temperature object, but it cannot be transferred spontaneously from a low-temperature object to a high-temperature object. Similar to the "reverse operation" of a water pump, a heat pump device can consume a certain amount of mechanical energy to "pump" heat from a low-temperature environment to a high-temperature environment. This is the temperature control principle of ordinary air conditioners. According to the way the heat pump is driven, the heat pump can be divided into electric drive, thermal energy drive and engine drive.

Figure 2 Gas heat pump industrialization demonstration project

(Image source: Website of Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences)

Unlike the electric-driven compressor in ordinary electric air conditioners, the first domestic gas heat pump (GHP) developed by the Energy Storage Technology Research Laboratory of the Guangzhou Institute of Energy Conservation, Chinese Academy of Sciences, is an ultra-high energy-efficiency air source heat pump system that mainly uses an open scroll compressor driven by a gas engine. Its complex thermal system coupling control technology and system integration have broken the technological monopoly of Japanese companies in this field, achieved localization, and will provide support for the realization of the "dual carbon goals."

Figure 3 Gas heat pump outdoor unit

(Image source: Website of Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences)

Figure 4 Schematic diagram of GHP system

(Image source: provided by the author)

In addition to the controller system, GHP includes four subsystems, namely the power system, heat pump system, circulating water system and waste heat recovery system. The power system is mainly composed of a natural gas engine and a pulley. The heat pump system mainly includes an open scroll compressor, a condenser, an evaporator and a throttle valve. The circulating water system recovers the heat in the refrigerant when the indoor heat exchanger is a condenser, and recovers the cold in the refrigerant when the indoor heat exchanger is an evaporator. The waste heat recovery system not only recovers the heat in the cylinder jacket water of the natural gas engine, but also recovers the heat in the flue gas, making the flue gas emission temperature lower than 100°C.

After natural gas burns in the gas engine, about 30% to 35% of the heat is converted into shaft work and driven by a multi-V belt scroll compressor with extremely high mechanical efficiency, so that the R410A refrigerant in it undergoes a vapor compression cycle. Most of the remaining 65% to 70% of the heat is transferred to the circulating water system by the waste heat recovery system. The rated heating capacity of the unit is 85kW, the maximum heating capacity exceeds 100kW, and the rated cooling capacity is 71kW.

The role of gas heat pump

In the hot summer, the temperature inside the room is lower than that outside, so how does the heat escape from the room to the outdoor environment?

This is because the refrigerant in the heat pump system is compressed into a high-temperature and high-pressure gas by the compressor, and is in a high-temperature state relative to the ambient temperature in the outdoor heat exchanger. At this time, its heat is spontaneously released to the environment and becomes liquid refrigerant. After being reduced and throttled by the electronic expansion valve, it becomes a low-pressure and low-temperature two-phase refrigerant and then flows into the indoor heat exchanger. After absorbing the heat in the circulating water, it completely evaporates into a gaseous refrigerant and then enters the compressor to start the next cycle. At the same time, the low-temperature circulating water exchanges heat with the indoor air and releases its coldness to the room.

Think about it, in the cold winter, the temperature inside the room is higher than that outside, so how does the heat get from the outdoor environment into the room?

This is because the refrigerant in the heat pump system is compressed into a high-temperature and high-pressure gas by the compressor, and then transfers the heat to the circulating water in the indoor heat exchanger. The circulating water is heated and exchanges heat with the indoor air, releasing its heat to the room. At the same time, the refrigerant changes from gas to liquid, and after passing through the electronic expansion valve to reduce the pressure and throttle, it becomes a low-pressure and low-temperature two-phase state and then flows into the outdoor heat exchanger. After absorbing the heat in the environment and completely evaporating, it enters the compressor to start the next cycle.

Are there any more powerful features besides this?

Yes, there is! With the unique waste heat recovery system in GHP, 25~30kW of domestic hot water can be obtained for different scenarios while cooling and heating, and even heating can be achieved in extremely low temperature environments, which is incomparable to electric heat pumps. The heating performance is not affected at an ambient temperature of -10℃, and defrosting can be achieved without stopping.

Figure 5 Applicable scenarios of gas heat pumps

(Image source: provided by the author)

Advantages of gas heat pumps

Compared with electric heat pumps (EHP), GHP has a very high primary energy utilization rate. Taking winter heating as an example, let's see what happens to one unit of energy in EHP and GHP?

In simple terms, whether it is EHP or GHP, the low-grade heat obtained from the air is equivalent, but compared with EHP, GHP directly utilizes the waste heat after natural gas combustion, so that the thermal economic value returns to the user. According to calculations, under the same output heating effect, the primary energy consumed by using EHP will be 1.36 times that of using GHP.

Figure 6 Comparison of GHP and EHP energy flows

(Image source: provided by the author)

At present, the main source of my country's power resources still relies on thermal power generation, and the intermittent and volatile nature of "green electricity" is still a problem that cannot be ignored. In other words, renewable energy is still not up to the task in the short term. According to statistics, natural gas, as a clean energy, will reduce carbon dioxide emissions by 200 million tons for every 1% replacement of coal. At the same time, it has a trend opposite to the peak-to-valley difference of power grid load. In the context of HVAC energy consumption accounting for 15% of the total social energy consumption, GHP can greatly reduce the distribution capacity and play a positive role in "shifting peaks and filling valleys".

Taking Chongqing as an example, it is its relatively cheap natural gas resources that have effectively smoothed out the "peak" months of electricity demand.

Figure 7 Power generation and natural gas supply in Chongqing in 2021

(Photo source: National Bureau of Statistics Chongqing Municipal Economic and Information Commission)

The gas source of GHP is not only natural gas, but also "green gas" such as biogas and biomass gas. Taking natural gas as an example, when GHP is used to replace gas boilers, gas consumption and carbon emissions are reduced by more than 40%. When used in heat pump air conditioners, the CO2 emitted for each kilowatt-hour of electricity consumed is calculated as 0.79 kg. Under the same circumstances, the carbon emissions of GHP are 60% of those of EHP.

Just think about it, in rural areas where biogas is abundant, the CO2 after combustion can also be transported to greenhouses. Isn’t that beautiful!

Editor: Sun Chenyu

References:

[1] Jiayao Tian, ​​Yafei Hu, Jie Lv, Ziping Feng, Wenji Song. Modeling and performance analysis of power system in gas engine-driven heat pump, Applied Thermal Engineering (2023) DOI: https://doi.org/10.1016/j.applthermaleng.2023.120015

[2] Roselli C, Marrasso E, Sasso M. Gas Engine-Driven Heat Pumps for Small-Scale Applications: State-of-the-Art and Future Perspectives[J]. Energies, 2021, 14.

[3] Y. Hu, J. Lv, C. Huang, W. Song, Z. Feng, Experimental investigation on heating performance of a gas engine-driven heat pump system with R410A, Applied Thermal Engineering (2022)

doi:https://doi.org/10.1016/j.applthermaleng.2022.118863

[4] Hu Yafei, Lü Jie, Han Tao, Song Wenji, Feng Ziping. High-temperature heating characteristics of gas heat pump system based on waste heat recovery[J/OL]. Chemical Industry Progress: 1-17[2022-07-15].DOI:10.16085/j.issn.1000-6613.2021-1869.

[5] Sanaye S, Chahartaghi M. Thermal modeling and operating tests for the gas engine-driven heat pump systems[J]. Energy, 2010, 35(1):351-363.