Method for rating energy performance of public buildings

Abstract

This paper discussed a comparative study of several state-of-art methods for determining building energy consumption benchmark. A new approach, which combined the idea of “building benchmark” and “operational benchmark” in its rating system, was proposed. A case study was conducted which applied the proposed approach to benchmarking an existing office building in Tianjin. Besides, the calculation of benchmarks of the reference building model and real building model using the rating method in eQUEST was also considered. Furthermore, the simulation results of the reference building model were taken as the baseline to divide real office buildings into different energy performance grades.

Keywords

energy consumption benchmark / energy efficiency benchmark / energy performance level

Introduction

Along with the expansion of urban construction and rapid development in China, the energy consumption in public buildings extensively increases in recent decades. Thus, it is urgent to find out an effective method to control the energy consumption [1]. At present, many researchers in China [2–5] focus on and make achievements in the fields of public building energy consumption benchmark and building energy-saving design, while the Chinese government has established standards such as “Evaluation Standard of Green Building” and “ Design Standard for Energy of Public Buildings ” [6] (hereinafter referred to as DSEPB) to enhance building energy efficiency. Some foreign researchers [7–9] have made great contributions to the development of building energy consumption evaluation methods, the most representative of which is “Energy Star Benchmarking Tool” developed by US Environmental Protection Agency. The Energy Star Benchmarking Tool can assess the building energy consumption characteristics and describe the impact of architecture on energy use by monitoring real building energy bills, setting up building energy consumption indicators, and making comparison with similar buildings [10]. However, those studies focus merely on the requirements of certain indicators or the building energy consumption, without establishing a comprehensive evaluation method for the two aspects. In this paper, various building energy consumption benchmark rating systems were compared. A comprehensive method to evaluate building energy efficiency was proposed and an office building energy performance rating method was established.

Method

Building energy consumption benchmark and energy efficiency benchmark

The energy consumption benchmark and energy efficiency benchmark should be established first in order to assess the building energy performance. Weighting integral thermal performance of building envelope, introduced by DSEPB [6], is one way to set energy consumption benchmark in the building design phase. Currently, statistical analysis methods, such as mean value method, regression analysis method and fixed horizontal method, and technical calculation method, as shown in Table 1, are the main methods for determining energy consumption benchmark in the building operation phase.

Through the comparison and analysis of different methods for determining building energy consumption benchmark, Building Benchmark (= signs E_B) and Operation Benchmark (= signs E_O) were provided to establish building energy consumption benchmark [11]. E_B, which reflects the internal thermal performance of building envelope, represents the annual required quantity of cold/heat on the condition that building indoor environment meets the requirements of DSEPB indoor calculation parameters without interior load. E_O, which mainly signifies building energy use, represents the annual energy consumption under the circumstances that the building accords to DSEPB requirements with the interior loads (such as anthropogenic heat, lighting and equipment loads) and air-conditioning heating system. Then, these two indicators were combined and E_B/E_O was adopted to suggest energy efficiency benchmark of a specified type of building, which synthetically evaluated the influence of various factors on building energy efficiency. Similarly, E_C, E_R and E_C/E_R were used to indicate the energy consumption benchmark, energy efficiency benchmark and energy efficiency of the real building, respectively.

Due to the lack of detailed building energy consumption statistics in China, it becomes essential to use energy simulation software to build the energy analysis model as reference and calculate its benchmarks as the baseline for the same type of building. Therefore, the energy performance of the real building can be estimated by comparing its energy efficiency value with its reference model. As for the formation method for reference model, in this paper, the real building was regarded as the prototype according to the criteria of DSEPB standard. The heat transfer coefficient of each part of the building envelope was then adjusted to its limits under the conditions that the window-wall ratio is identical to the real building. Finally, activity schedules, such as personnel density, equipment power density, running time and lighting power density, were established in light of the Appendix B in DSEPB.

Analysis of the factors affecting energy consumption

Factors that have a significant impact on building energy consumption can be divided into two categories: hardware factors and software factors [5]. Hardware factors primarily include building thermal characteristics (such as building orientation, shape factor, window-wall ratio, building envelope thermal resistance and external window shading coefficient, etc.) and equipment performance (mainly study on the energy performance of air-conditioning heating system here). Software factors primarily involve building operation modes (such as lighting power density, equipment power density, personnel density and running time), management level, and energy conservation awareness. Moreover, Building Benchmark is largely affected by building thermal characteristics while Operation Benchmark is simultaneously influenced by these two factors.

Buildings, with the same thermal characteristics and equipment performance, may consume energies differently owing to different operation modes. For example, building, with high personnel density and long running time, may consume more energy, resulting in a higher operation benchmark than the average value, but it cannot be classified into the low energy efficient category. Therefore, before assessing building energy performance, the characteristic of the real building need to be adjusted and the operation modes kept the same as the reference one for the benefit of comparison of the same kinds.

Assessment of energy performance of office building

Descriptions of case study building

There are varieties of public buildings, each of which has its own characteristics. This paper merely focuses on office buildings and provides a valid method for assessing its energy performance. The building for the case study is an office building in Tianjin whose information is detailed in Tables 2 and 3. The design heating indoor temperature of conditioned rooms is 16°C or 18°C depending on room types, and design cooling indoor temperature for all conditioned rooms is 26°C. Its design cooling load is 354 kW and design heat load is 160 kW. The heat transfer coefficient of each part of the building envelope is smaller than the limits specified in DSEPB. This building uses renewable energy as the cold/heat source. Moreover, it assumes independent temperature-humidity control system using outside fresh air for the indoor latent heat load and air conditioning terminal device for sensible heat/cooling load.

Energy consumption calculation method in eQUEST

The input to eQUEST consists of a detailed description of the building being analyzed, including the hourly scheduling of occupants, lighting, equipment, and thermostat settings. eQUEST provides very accurate simulation of such building features as shading, interior building mass, envelope building mass, and the dynamic response of differing heating and air conditioning system types and controls. eQUEST calculates hour-by-hour building energy consumption over an entire year (8760 h) using hourly weather data for the location under consideration with the method of dynamic calculation assuming that each space is kept at a constant user-specified temperature. The heat gain/loss, which changes with outdoor climate conditions and the heat transfer through the building envelope, is calculated by using the response factor method. Therefore, the annual required quantity of cold/heat of the building can be calculated with positive thinking which is applied to calculate the indoor temperature and indoor heat gain. The calculating process is a process of dynamic equilibrium in which indoor temperature, cooling/heating load and power consumption of the air-conditioning system are affected by the status of the building before that moment. Finally, eQUEST can list the total annual energy consumption and annual energy consumption of the building by end users including space cool, heat reject, refrigeration, space heat, etc.

Energy consumption benchmark of the reference building

Based on the shape, dimension and window-wall ratio of the real building, the heat transfer coefficient of each part of the reference building model is set up according to Table 3, and the “Limits of the heat transfer coefficient specified in DSEPB” is built up in the eQUEST simulation software. The design heating indoor temperature of conditioned rooms is 18°C or 20°C depending on room types, and design cooling indoor temperature for all conditioned rooms is 25°C [5]. The calculated annual required cold/ heat quantity, namely, building benchmark of the reference building (E_B) is 115.71 kW·h/m².

When calculating the operation benchmark of the reference building, the operation modes are defined according to Tables 4 and 5.

Heating/cooling system energy consumption is an important part of the total energy consumption, varying with the form of air conditioning and heating system. On one hand, systems in different forms correspond to different energy efficiencies and energy conversion efficiencies; on the other hand, cold/heat source is a significant factor on heating/cooling system. Therefore, five most representative kinds of cold/heat sources with the corresponding three levels of energy efficiency are considered in simulating the annual energy consumption of the reference building [12]. The authors will try to determine the operation benchmark of the reference building accurately with the specific conditions, shown in Table 6.

The cold source energy efficiency level refers to the following national standards: the Minimum Allowable Values of the Energy Efficiency and Energy Efficiency Grades for Water Chillers GB19577-2004; the Minimum Allowable Values of the IPLV and Energy Efficiency Grades for Multi-connected air-condition (heat pump) unit GB21454-2008; the Minimum Allowable Values of the Energy Efficiency and the Energy Efficiency Grades for Lithium Bromide Absorption Chillers GB29540-2013. The heat source energy efficiency level is determined according to the above national standards when the cold source and the heat source are the same or when the Minimum Allowable Values of the Energy Efficiency and Energy Efficiency Grades of Industrial Boilers GB24500-2009 is set individually. If taking the district heating system as the heat source, because of the energy loss in the transmission process, its energy efficiency should be set to 90% based on the Energy Conservation Design Standard for New Heating Residential Buildings JGJ26-95.

In all the types in Table 6, except the fifth, the air conditioning terminal devices are primary air fan-coil system. The fresh air in all air-conditioned rooms is 30 m³/(h·person). The circulation pumps in the heating and cooling systems are set separately and run frequently. The operation benchmark simulation results of the reference building at different energy efficiency levels are shown in Fig. 1. Furthermore, coal and gas consumption have been equally converted into electric consumption on the principle that their caloric value are the same in the simulation results.

Figure 1 shows that the amount of annual energy consumption of buildings from Type 1 to Type 5 is gradually increasing and that using renewable energy as cold/heat source contributes significantly to building energy-saving. To determine the operation benchmark of the reference building (E_O), this paper temporarily chooses the upper quartile, 104.45 kW·h/m², from all the simulation results as the ideal one, which is benefit for the realization of energy conservation and accords with the actual energy consumption situation. In this paper, the baseline is set from rationality and operability. If the baseline is too low, it will not get expectant energy-saving effect. If the baseline is too high, it will result in the fact that most of the buildings cannot reach the baseline, increasing the difficulty of operation and implementation. Though the method of setting the baseline in this paper is a little rough, the baseline exists only as entry level and will be reset more precisely after a further study in practice.

It is necessary to establish different reference buildings to evaluate energy performance given the fact that the design cooling and heat load of the real office buildings in different climate zones are quite different. However, the operation benchmark of the office reference buildings in the same climate zone within a fixed design cooling and heat load range are the same. The reasons for this are that, first, it mainly reflects the energy consumption baseline value of energy use system, taking air conditioning, heating system as the first object of research; second, the limits of the heat transfer coefficient are the same in the same climate zone. Considering the individual difference among real buildings, the reference building should be re-established in line with the shape and dimension of the real building so as to make the energy efficiency benchmark value more reliable and accurate. The energy efficiency benchmark of the reference building (E_B/E_O) is 1.09 in this paper, which can only be the baseline of the real office buildings located in the cold region and of which the design cooling and heat load are in a certain range. The energy efficient baseline values of other buildings have to be recalculated.

Energy consumption benchmark of the building for case study

Calculated by the eQUEST simulation software, the building benchmark of the building (E_C) for case study is 101.15 kW·h/m².

Given that the operation mode of the real building is not exactly the same with the reference building, it should be adjusted to the standard uiliztion pattern as listed in Tables 4 and 5 when determining its operation benchmark. However, the hardware factors (building thermal characteristics, equipment performance) cannot be amended, because they act as the decisive factors in determining energy consumption benchmark and demonstrate the importance of initial investment and decision-making. Since software cannot simulate the capillary system settled in real building, the primary air fan-coil system is used to replace it when eQUEST calculates the annual energy consumption of the corrected real building. It should be noted that the design heating indoor temperature of conditioned rooms have to be decreased by 2°C and design cooling indoor temperature by 1°C in simulation to make the model similar to the actual situation [13]. As a result, the operation benchmark of the real building (E_R) is 69.13 kW·h/m², and its energy efficiency (E_C/E_R) is approximately 1.46.

Energy performance rating of office building

After the energy consumption benchmark values of the real building and the reference building are obtained, the assessment of energy performance of office buildings can be conducted from two aspects. First, a comparison of the building benchmark between real building and reference building should be made. If E_C is less than or equal to E_B, the integral thermal performance of real building envelope is good and meets the energy-saving requirement of the DSEPB and vice versa. Second, the relative energy efficiency ratio

{\displaystyle \frac{E_{\mathrm{C}}/E_{\mathrm{R}}}{E_{\mathrm{B}}/E_{\mathrm{O}}}}

should be used to denote the energy performance of the real building. E_C/E_R represents the ratio of energy demand to consumption, which reflects the efficiency of energy utilization system of the real building under the circumstances that indoor environment meets the DSEPB standards with the building envelope of certain thermal performance. The larger E_C/E_R is, the less energy the building consumes, and the more efficient the energy use system is. E_B/E_O shows the energy utilization efficiency of the reference building, which is equivalent to the energy efficiency baseline of the real building. Larger relative energy efficiency ratio represents higher energy efficiency of the real building. The formula for evaluation of the energy performance is defined as shown in Eq. (1).

(1)C={\displaystyle \frac{E_{\mathrm{C}}/E_{\mathrm{R}}}{E_{\mathrm{B}}/E_{\mathrm{O}}}},

where C is the energy performance grading parameter; E_C, the energy consumption benchmark of the real building; E_R, the energy efficiency benchmark of the real building; E_B, the building benchmark of the reference building; and E_O, the operation benchmark of the reference building.

Apparently, parameter C not only numerically equals relative energy efficiency ratio but also has the same physical meaning with

{\displaystyle \frac{E_{\mathrm{C}}/E_{\mathrm{R}}}{E_{\mathrm{B}}/E_{\mathrm{O}}}}

stated before. Taking the energy efficiency benchmark of the reference building as the baseline, the energy performance of the real building can be grouped into three levels. Larger grading parameter value C means higher energy performance level and energy efficiency of the real building [14], as tabulated in Table 7. The energy performance level is divided by taking the energy efficiency benchmark value of reference building as the baseline. Therefore, the real building can be classified as the first grade when its energy efficiency value is less than the benchmark value, or the second, or the third grade when higher. Although this rating system is ambiguous, it is a qualitative theory. In practice, the system could be rebuilt based on a large number of samples.

When the integral thermal performance of real office building envelope is poor and its energy performance level is lower than grade 2, it is better to find the reason for high energy consumption and implement energy conservation transformation. When the integral thermal performance of the real office building envelop is good and the energy performance level is higher than grade 2, the government should give awards to the owners to encourage energy conservation activities. As for other assessment results, buildings can remain unchanged. The energy performance grading parameter of the building for case study is equal to 1.32 and E_C<E_B, Therefore, it belongs to the first grade.

Conclusions

This paper compared and anatomized various methods for determining building energy consumption benchmark and provided a new approach that made use of “building benchmark” and “operation benchmark” to build energy consumption benchmark and energy efficiency benchmark. The reference building energy efficiency benchmark of a real office building in Tianjin was calculated and treated as the baseline value in the case study. Furthermore, this paper introduced a method for rating the energy performance of real buildings, especially office buildings.

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