Scheme design and economic analysis of an office building’s cold and heat source system in hot-summer and cold-winter zone

In the context of green development and sustainable development, in response to the national policy of “carbon neutrality and peak carbon dioxide emissions”, we will fully implement clean energy heating and reduce air pollution. Air energy is the representative, bringing the comfortable, efficient, safe and energy-saving environment to people. For an office building in Nanjing, which is located in hot-summer and cold-winter zone, an air source heat pump unit is designed as a cold and heat source system to meet the demand for heating in winter and cooling in summer. At the same time, compared with the traditional cold and heat source system which is designed central heating in winter and multi-line refrigeration in summer, the result of economic analysis shows that the use of air source heat pump as the cold and heat source system of office buildings is more economical and energy-saving. It is suitable to popularize and apply in China’s hot-summer and cold-winter zone.


Introduction
The air source heat pump unit uses the energy in the air as the main power. It has the characteristics of simple system structure, convenient installation, high degree of automation and utilization of renewable energy And it is widely used at home and abroad. In the actual application process, with its high work efficiency, it gradually reduces the large amount of pollutant emissions brought by traditional heating methods to the atmosphere, which has been recognized by many people in the industry. Nanjing is located in China's hot-summer and cold-winter zone. There is no central heating in winter,so air source heat pump is a better option. In this paper, the air source heat pump is used as the cold and heat source system to conduct HVAC design and economic analysis for an office building in Nanjing.

Project Overview
This project is an energy-saving office building located in Nanjing, Jiangsu Province.The building area is 6000m 2 , designed according to the building energy saving 65%. The office hours of the office building are from 9:00 to 20:00. The calculation parameters of outdoor air in Nanjing are shown in Table 1 [1] .

Table1. The calculation parameters of outdoor air in
Nanjing.
The outdoor design temperature for heating(℃) -1.8 The outdoor design dry-bulb temperature for summer air conditioning(℃) 34.8 The extreme minimum temperature(℃) -13.1 The number of days when the average daily temperature is ≤+8℃(d) 109

Estimating heating load in winter and cooling load in summer
According to Table 3.1.2-1 in "Design code for city heating network"(CJJ34-2010), the recommended value range of heating index for office buildings with energysaving measures is 50~70 W/m 2 , and the recommended value range of cooling index is 80~110 W/m 2 [2] . Therefore, for an energy-saving office building in Hefei, Anhui Province, the heating index in winter is 50 W/m 2 , and the cooling index in summer is 90W/m 2 . Since the building area is 6000m 2 , the heating load in winter is Q 1 =6 000×50 W/m 2 =300 kW, and the cooling load in summer is Q 2 =6 000×90 W/m2=540 kW.

Using existing heat pump samples to select models
The heating effect of the air source heat pump unit is given priority.

Selecting auxiliary heat source
In extreme weather, the heating capacity of the air source heat pump unit will be affected, and an auxiliary heat source should be provided. The capacity of the auxiliary heating device is determined according to the difference between the effective heat generated by the air source heat pump unit and the heat consumption of the building under extreme weather conditions in winter. In order to ensure that the transformer installed capacity of the entire building does not increase due to the use of electric heating in winter, it is required that the direct electrical heating load in winter does not exceed 20% of the cooling load in summer, and the total installed direct electrical heating capacity per unit building area does not exceed 20W/m² [3] . The auxiliary heat source of this design adopts electric heating boiler. As the extreme minimum temperature below -10℃ rarely occurs in Nanjing, and the low temperature weather events in winter show a significant downward trend [4] , the number of days when the extreme minimum temperature is considered according to the most unfavourable conditions should not exceed 2 days.
Since the unit selection in winter is based on the ambient temperature of -4℃, the total heating capacity of the two units is 196 (=98×2) kW in the case of reduced efficiency in extreme weather, which cannot meet the building's heat consumption of 300kW. It can be calculated that the difference between the effective heat generated by the air source heat pump unit and the heat consumption of the building under extreme weather conditions in winter is 104(=300-196) kW.
Therefore, when running 2 air source heat pump units, 104kW electric heating boiler is equipped. 20% of the building cooling load in summer is 108kW, and the total installed capacity of direct electrical heating per unit building area is 17.3W/m². Equipped with a 104kW electric heating boiler, it can not only meet the requirements of the specification that the direct electrical heating load in winter does not exceed 20% of the building cooling load in summer, but also meet the requirement that the total installed capacity of direct electrical heating per unit building area does not exceed 20W/m².
Calculation formula for power consumption of electric auxiliary heating: W is power consumption (kW). h is the thermal efficiency of water heating equipment (90%～95%). Then W =104/95%=109.5 kW.
Therefore, during the extreme weather period (usually calculated as 2days, 11h per day), the total power consumption that needs electric assistance is Q =109.5×2×11=2409 kW·h.

Initial investment of unit
The model parameters of the air source heat pump unit selected in this office building are shown in Table 3. The cost of the air source heat pump unit is 1.5 yuan/W, so the initial investment of the selected MHS F ST2 060.1 units is 92.25 (ten thousand yuan).
The cost of the electric-assisted boiler is 0.35 yuan/W, so the initial investment of the electric-assisted boiler is 3.64 (ten thousand yuan).
In summary, the initial investment is 95.89 (ten thousand yuan).

Operating costs of heat pumps in winter
According to the office hours of the office building, the operating time of the unit in winter is 11h (8:00-19:00). Considering the number of days when the average daily temperature ≤ +8℃ is 109 days, the heating period is 109 days. According to the electricity price list of Jiangsu Electricity Grid, the electricity price for general industrial and commercial use is 0.6414 yuan/kWh. According to the time-sharing sales of electric boilers in Jiangsu Province, the electricity price of electric boilers in general industrial and commercial sectors (8:00-24:00) is 0.6414 yuan/kWh [5] .
See Table 4 for the calculation results of heat pump operating costs in winter.

Total cost of central heating
At present, there is no urban central heating in Nanjing. Refer to Zhengzhou urban infrastructure supporting fee collection management method, the heating pipe network construction fee is 60 yuan/m 2 [6] , the initial investment cost of central heating is the product of the unit area heating pipe network construction fee and building area, 36 (ten thousand yuan). With reference to the "Administrative Measures for Urban Heat Supply and Heat Consumption of Zhengzhou" and the relevant documents of Zhengzhou City Price Bureau, the unit price of central heating in winter is 0.19 yuan/m 2 ·day [7] . Since the heating period is 109 days and the unit price of central heating in winter is 20.71 yuan/m 2 . The total cost of central heating in winter is 12.4 (ten thousand yuan).
In summary, the calculation results are summarized in Table 5. It can be seen from Table 5 that the operating cost of air source heat pumps is lower than that of central heating.

Economic analysis in summer
According to the office hours of the office building, the number of cooling days is 135 days.

Operating costs of heat pumps in summer
See Table 6 for the calculation results of heat pump operating costs in summer.

Total cost for varied refrigerant volume
The initial investment per unit area of varied refrigerant volume is about 200 yuan/m 2 , and the construction area of this office building is 6000 m 2 . So the initial investment of the multi-line is 120 (ten thousand yuan). See Table 7 for the calculation results of the total summer cost for varied refrigerant volume.

Comparative economic analysis
The air source heat pump unit is used as the cold and heat source system, and the central heating and varied refrigerant volume are used as the cold and heat source system for comparative analysis. The data results are summarized in Table 9. It can be seen from the data that the initial investment cost of central heating in winter and varied refrigerant volume in summer is 601,000 yuan more than that of air source heat pumps. Although the operating cost of the air source heat pump unit is extra 6,000 yuan per year, the initial investment of central heating and varied refrigerant volume cannot be fully covered within the service life of varied refrigerant volume for 20 years. If combined with Jiangsu Province's air source heat pump promotion policy, the operating cost of air source heat pump units will be lower.

Conclusion
Air source heat pump units are used as the cold and heat source system for office buildings in Nanjing, which is located in China's hot-summer and cold-winter zone, which can meet its needs with heating in winter and cooling in summer.
Comparing the traditional central heating in winter and the varied refrigerant volume cooling mode in summer with the air source heat pump unit as the cold and heat source system, it is found that the air source heat pump unit is a more economical and energy-saving choice. In winter,the outdoor design temperature for heating is used as the basis for selection of air source heat pump units, and the initial investment of the unit is only 958,900 yuan. Although it is necessary to add an electric auxiliary heating boiler, compared with the use of extreme minimum temperature as the basis for unit selection, it avoids the choice of oversized units, saves energy and reduces initial investment. Therefore, the use of air source heat pumps as the cold and heat source system of office buildings is more economical and energy-saving, and it is suitable for popularization and application in China's hotsummer and cold-winter zone.