Low-carbon Transformation of Nankai Campus Based on Energy Audits Comprehensive utilization of New Energy

. The existing campus has a large building area, a low level of energy saving, less renewable energy utilization, higher energy consumption, and obvious energy-saving space. Based on the energy audit of Nankai University, this paper proposes a low-carbon campus transformation technology system combining building energy saving, clean heat supply and distributed photovoltaic in view of its problems such as high municipal heating energy consumption, low comfort, high electricity bill, and tight electricity consumption. Because the campus has both winter holidays and there is tidal phenomenon in teaching buildings and dormitory buildings, etc., the heating demand is reduced, so its heating and energy-saving space is large. At the same time, the number of campus building floors is low, the roof is conducive to the installation of photovoltaic, it is estimated that the annual power generation of rooftop photovoltaic installation can reach more than 70% of the campus electricity consumption, combined with energy-saving transformation and smart energy management, it is easy to achieve its low-carbon goals.


Introduction
University is the region with population, energy facilities, energy density, and energy and resource consumption higher than the average level of society [1] . In order to promote energy conservation and emission reduction in education system, on October 26, 2022, the Ministry of Education of China issued the Implementation Plan for the Construction of National Education System for Green and Low-Carbon Development.
Davis has used PV, biomass co-generation, biomass boilers and heat pumps as energy alternative systems to build a carbon-neutral campus [2] , the University of Dayton has proposed four major emission reduction strategies: energy-efficient energy systems, electrified transportation,geothermal heat pumps and clean electricity, to build a carbon-neutral energy system. The University of Palermo puts forward the low-carbon transformation path of campus from three aspects of construction, transportation and personnel flow [3] . Mata [4] comprehensively evaluated the energy performance of university campuses through the campus decay technology index [5] . The carbon emissions of campus buildings [6] , the construction of low-carbon campus is promoted by the consumption characteristics, lifestyle and low-carbon consciousness of college students [7] . The method of carbon balance accounting to explore the realization path of zero-carbon campus [8] [9] . Due to the high greening rate, the annual carbon sequestration of campus is greater than the living carbon emissions of teachers and students [10] .
Aim to build the low carbon campus, the energy efficiency improvement and electric energy substitution, the clean heating, and distributed roof PV and energy storage facilities on a large scale to meet all the electricity demand on campus of Nankai University were proposed in this paper.

Campus Overview
Balitai Campus of Nankai University built in 1923, located in Nankai District, Tianjin, covering an area of 1216,000 m 2 with a building area of 677,500m 2 and 17360 students. The old building area of the campus accounts for about 32.59% before the 1980s, see figure 1. From the perspective of the proportion of comprehensive energy consumption, municipal heat accounts for about 60% of the annual energy consumption, followed by electricity accounting for about 30%, the third is natural gas, accounting for about 8%, and gasoline and diesel account for a very small proportion.

Characteristics of campus energy consumption
The energy consumption of campus operation in 2017-2019 is shown in Table 1. From 2017 to 2019, the total energy consumption of the campus showed an overall upward trend, 4.55% in 2018 and 2.86% in 2019. Among them, the consumption of electricity and municipal heat accounts for the highest proportion. From 2017 to 2019, the electricity annual growth rate is 4.41%, see figure2. The 2020 is not normal to be affected by COVID-19, electricity has become the main part of campus energy consumption, which provides greater feasibility for the use of PV and energy efficiency improvement technology.

Carbon accounting based on energy balance
The construction of "zero-carbon campus" needs quantitative analysis to ensure the effectiveness of carbon reduction strategies. The research is based on energy balance, to calculate and analyze the consumption of all kinds of energy on campus from the "carbon source" side, and to calculate the total carbon emissions. On the "carbon sink" side, the potential of renewable energy utilization on campus is analyzed, see Table2.

Energy consumption benchmark
Based on the Chinese National Standard of 'Energy and Resource consumption quota of central and state organs' , combined with the existing energy consumption of the campus, the indicators of heating and non-heating energy consumption and per student energy consumption of different types of buildings are proposed to guide the energy-saving transformation of building units and the overall energy consumption control of campus, see Table3.

Energy consumption and carbon emission accounting
Carbon emissions per unit building area showed in Table  4. The benchmark value of carbon emissions per unit building area of comprehensive colleges and universities is 47.08 kgCO2/m 2 , the carbon emissions per student on campus is 1129.00 kgCO2/p, and the average emission per unit building area of Balitai Campus in 2017-2019 is more than twice the benchmark value. Carbon emissions per inhabitant are more than twice the baseline value. The campus has a large space for carbon emission reduction.

New energy system design
From the energy consumption situation and the problems found in the energy audit, the campus energy system design is carried out based on the energy consumption bench-marking, refined load analysis, and energy resource availability assessment, as shown in the figure 3. Based on the evaluation of campus building energy consumption, energy supply system energy efficiency and available energy resources, based on the principles of passive priority, active optimization and maximum utilization of renewable energy, combined with multifactor evaluation, the appropriate energy system mode is selected, among which the more typical technical paths are building energy-saving transformation, clean heat supply, distributed photovoltaic and smart energy management.

Distributed PV installation capacity assessment
Due to the large number of campus buildings, it is difficult to adopt the building-by-building evaluation method, so a campus photovoltaic evaluation method based on UAV, parametric design, photovoltaic software measurement, etc. is proposed. First collecting the basic data of each functional group building and generating a three-dimensional geometric model in a parametric software, applying a parameterize method to analyze and calculate the available photovoltaic area of the roof of the campus building, a suitable photovoltaic matrix deployment mode and obtain the power generation potential; According to the monthly and hourly electricity load of each functional group on campus, the photovoltaic substitution scenario is predicted and the photovoltaic deployment strategy is formulated, see figure 4.

Evaluation process and results
According to the building area data of Balitai Campus, the building roof area of Balitai Campus is calculated and corrected by extracting the building outline through the online map and ENVI software, and the building area of some old one-storey buildings and one buildings is deleted. It is estimated that the area of the roof where PV can be installed is about 104000m 2 , The best inclination angle for installation is 39.1°, the PV panels installed in the campus capacity is 20571.54kWp, the total power generation is simulated is about 25225437kWh, as shown in Figure 5    In order to meet all the energy consumption of campus operation, it is necessary to consider installing PV in parking places. Through calculation, 32073 PV modules are required to be installed, with an installation area 53,500 m 2 . Based on the current building density 29.65% and the greening rate is 63.00%, only 4% of the open space of the campus is required to install PV modules.

Clean heating
Due to the low density of campus buildings, the existence of winter vacation, the complexity of campus building types, the obvious alternation of day and night in teaching and dormitory buildings, and the intermittent heating demand in some spaces, based on renewable energy, the regional energy planning and clean heating can greatly reduce building heating energy consumption. Taking the regional heating system based on air source heat pump as an example, according to the characteristics of heating, cooling and hot water supply in the campus, the buildings on the campus are scattered, most of the buildings are small, and the differences in use time are large, so it is adopt the decentralized energy system approach. It can realize 100% clean heating using renewable energy, and has strong feasibility. The heating cost can be reduced by about 35% by using solar energy with air source heat pump, the investment recovery period is about 6 years.

Energy-saving measures
According to the lighting fixture data of the main buildings provided by the existing campus property, there are about 16796 fluorescent lamps and other general lighting fixtures at present. The non-LED lamps in the main buildings of the school are replaced by LED lamps. The campus can save about 184,000 kWh of electricity per year and reduce carbon emissions by 111.14 tCO2.
In addition, campus carbon emissions can be further reduced by electric vehicle, solar water heating system, intelligent transformation of electric water heater, recycling reclaimed water, reducing water consumption, reducing catering energy consumption.

Conclusion
This paper analyzes the energy consumption characteristics of the Balitai Campus of Nankai University, based on the energy balance, puts forward the construction path of "zero carbon campus". According to the development and comfortable living need, especially the domestic hot water, electrical applications, the power consumption of campus is increasing, but the annual growth rate of total energy consumption is slowing down, and the PV installation price has decrease, which provides the possibility of power supply for the campus through renewable energy technology, with great application potential and considerable construction prospects. Renewable energy is calculated from the perspective of energy balance. The annual energy consumption of the campus is converted, and then the photovoltaic power generation technology is adopted, considering the photovoltaic areas such as building roofs and open space of the campus. Building roof photovoltaic installation will generate about 25.22 MWh of electricity in the whole year, accounting for 56% of the campus power consumption..