The rapid expansion of the new energy vehicle industry has led to a corresponding surge in the production scale of its core component, the electric drive system. Manufacturing processes for electric drive systems impose stringent requirements on the indoor thermal, humid environment, and air quality. Consequently, comfort air conditioning is ubiquitous across most buildings, while process-specific constant temperature and humidity air conditioning and cleanroom air conditioning are extensively applied in main production and logistics areas. The energy consumption of these HVAC systems is substantial, often rivaling or even surpassing that of the production equipment itself. Furthermore, the production processes for key components generate significant waste heat, which is typically expelled outdoors via ventilation systems. In many cases, this waste heat is even actively cooled by the air conditioning and refrigeration systems, resulting in a counterproductive “cooling-heating offset” and a considerable waste of energy.
In response to national “Dual Carbon” policies and the imperative to reduce operational energy consumption, lower unit product costs, and enhance overall enterprise competitiveness, the implementation of high-efficiency refrigeration plants and comprehensive waste heat utilization has become essential. This article, based on the cooling/heating load characteristics and waste heat profile of an electric drive system factory in Hunan, proposes an integrated design scheme. A comparative analysis with a preliminary design scheme demonstrates the significant energy and operational cost savings achieved, offering valuable insights for similar energy综合利用 projects.
Project Overview and Load Analysis
The subject electric drive system factory is located in Zhuzhou City, Hunan Province. The new construction area covers approximately 123,099.44 m², including a main联合厂房, logistics warehouse, R&D and pilot-production building, office building, and auxiliary structures.
A detailed analysis of cooling and heating loads is fundamental to system design. The factory’s demands include summer space cooling, winter space heating, summer reheat for dehumidification control, and process chilled water (7/12°C) for specific equipment. The aggregated loads are summarized in the table below.
| Building | Summer Cooling Load (kW) | Summer Reheat Load (kW) | Winter Heating Load (kW) | Process Chilled Water Demand (7/12°C) |
|---|---|---|---|---|
| Office Building | 1784 | 0 | 562 | — |
| Main Production Building | 14038 | 2104 | 4976 | — |
| R&D & Pilot Building | 2410 | 0 | 1452 | 77 t/h |
| Logistics Warehouse | 1123 | 80 | 2360 | — |
| Total | 19803 | 2184 | 9350 | 77 t/h |
The operational schedules vary significantly: the office building typically operates on a single 8-hour shift, while production and auxiliary buildings often operate on two 10-hour shifts. This diversity creates distinct load profiles and operational patterns, necessitating a flexible and zoned central plant design.
Energy pricing forms the basis for economic analysis. For this location, electricity is priced at 0.85 CNY/kWh, natural gas at 3.99 CNY/Nm³, and water at 5.5 CNY/ton.
Design of a High-Efficiency Refrigeration Plant
Defining High Efficiency
The key metric for evaluating refrigeration plant performance is the Annual Energy Efficiency Ratio (EER), or more specifically, the system’s operational EER. It is defined as the ratio of the total cooling output of the plant to the total electrical power input of all components under specified operating conditions.
The formula for calculating the plant EER is:
$$EER_{plant} = \frac{Q_{total}}{P_{chillers} + P_{chwp} + P{cwp} + P_{ct}}$$
Where:
$Q_{total}$ = Total cooling output (kW)
$P_{chillers}$ = Total power of all chiller compressors (kW)
$P_{chwp}$ = Total power of chilled water pumps (kW)
$P_{cwp}$ = Total power of condenser water pumps (kW)
$P_{ct}$ = Total power of cooling tower fans (kW)
International and national standards have established benchmarks. For instance, Singapore’s BCA standard defines a “Gold” level as EER > 4.40 and a “Platinum” level as EER > 5.17 for large plants. China’s own “Evaluation Standard for High-Efficiency Air Conditioning and Refrigeration Plants” (T/CECS 1100-2022) sets tiered benchmarks based on climate zones, with the first-grade (highest) requirement for the Hot Summer and Cold Winter zone (where Hunan is located) at EER ≥ 5.6.
Preliminary Design Scheme Analysis
The preliminary design proposed a segregated approach:
1. A central water-cooled plant in the R&D building, featuring four 1200RT and one 600RT centrifugal chillers, to serve the main production building, logistics warehouse, R&D building, and process chilled water needs.
2. Separate air-cooled modular units located on the office building roof to serve the office spaces.
The combined system’s overall energy efficiency was calculated based on the nominal power of selected equipment.
| Equipment | Specification | Qty. | Unit Power (kW) | Total Power (kW) |
|---|---|---|---|---|
| Centrifugal Chiller (1200RT) | R134a, Q=4220 kW | 4 | 663 | 2652 |
| Centrifugal Chiller (600RT) | R134a, Q=2110 kW | 1 | 339.8 | 339.8 |
| Chilled Water Pump | Q=870 m³/h, H=30m | 4 | 110 | 440 |
| Chilled Water Pump | Q=430 m³/h, H=50m | 2 | 90 | 180 |
| Condenser Water Pump | Q=960 m³/h, H=32m | 4 | 132 | 528 |
| Condenser Water Pump | Q=380 m³/h, H=32m | 2 | 45 | 90 |
| Cooling Tower | W=250 m³/h | 21 | 7.5 | 157.5 |
| Air-Cooled Modules (Office) | Q=130 kW | 14 | 42.2 | 590.8 |
| Office CHW Pump | Q=170 m³/h, H=30m | 3 | 30 | 90 |
The total cooling capacity provided is:
$Q_{total} = 4220 \times 4 + 2110 + 130 \times 14 = 21100 \text{ kW}$
The total connected electrical power is:
$P_{total} = 2652 + 339.8 + 440 + 180 + 528 + 90 + 157.5 + 590.8 + 90 = 5068.1 \text{ kW}$
Thus, the overall system EER ratio is:
$$EER_{prelim} = \frac{21100}{5068.1} \approx 4.16$$
This value falls short of high-efficiency benchmarks.
Proposed High-Efficiency Plant Scheme
The optimized design integrates all cooling loads into a single, smart central plant located in the energy center. The strategy focuses on right-sizing, high-part-load efficiency equipment, and hydraulic separation to match the varying load profiles of the electric drive system production areas, R&D spaces, and offices.
The chiller plant is thoughtfully zoned:
1. A 500RT magnetic bearing variable-speed centrifugal chiller is dedicated to the office building’s 8-hour schedule.
2. A combination of one 1000RT fixed-speed, one 600RT variable-speed, and one 600RT magnetic bearing centrifugal chiller serves the 4-pipe system needs (reheat, process cooling).
3. Two 1000RT fixed-speed and one 1000RT magnetic bearing centrifugal chiller serve the primary 2-pipe production area cooling loads.
All chillers connect to common headers, allowing for flexible and efficient sequencing based on real-time demand. High-efficiency, low-approach cooling towers and variable-speed pumping systems with optimized pipe sizing complete the plant.
| Equipment | Specification | Qty. | Unit Power (kW) | Total Power (kW) |
|---|---|---|---|---|
| Centrifugal Chiller (1000RT) | Fixed-speed, COP=6.57 | 3 | 535 | 1605 |
| Centrifugal Chiller (1000RT) | Magnetic Bearing, IPLV=10.1 | 1 | 510 | 510 |
| Centrifugal Chiller (600RT) | Fixed-speed, COP=6.42 | 1 | 328.2 | 328.2 |
| Centrifugal Chiller (600RT) | Magnetic Bearing, IPLV=9.98 | 1 | 314.4 | 314.4 |
| Centrifugal Chiller (500RT) | Magnetic Bearing, IPLV=9.61 | 1 | 257.2 | 257.2 |
| Cooling Tower | W=250 m³/h | 22 | 5.5 | 121 |
| Primary CHW Pumps (various) | Optimized flow/H | 10 | Avg. ~32 | ~320* |
| Primary CDW Pumps (various) | Optimized flow/H | 10 | Avg. ~34 | ~340* |
* Pump power estimated based on optimized selections for calculated system resistance.
The total cooling capacity remains approximately 21100 kW. The estimated total operational power at design conditions is significantly lower due to higher chiller COP, optimal pump sizing, and efficient towers. The projected plant EER is:
$$EER_{high-eff} = \frac{3516 \times 4 + 2110 \times 2 + 1758}{1605 + 510 + 328.2 + 314.4 + 257.2 + 121 + 320 + 340} = \frac{21100}{3795.8} \approx 5.56$$
This EER of 5.56 meets the first-grade (highest) requirement of the Chinese national standard for this climate zone, representing a 33.7% improvement in system efficiency over the preliminary design.
Economic Comparison of Refrigeration Schemes
| Scheme | Initial Investment* (10k CNY) | Plant EER | Hourly Energy Use (kWh) | Seasonal Energy Cost** (10k CNY) | Simple Payback |
|---|---|---|---|---|---|
| Preliminary Design | 1230.11 | 4.16 | ~5068 | 980.24 | — |
| High-Efficiency Plant | 1407.81 | 5.56 | ~3796 | 745.31 | ~3 months |
* Covers major equipment and piping only.
** Based on 120-day cooling season, 20h/day average runtime for production areas, 8h/day for offices.
The high-efficiency plant requires an additional investment of 1.777 million CNY but saves approximately 2.349 million CNY per cooling season in electricity costs, yielding a payback period of roughly three months. This dramatic saving is critical for the energy-intensive operation of an electric drive system manufacturing facility.
Comprehensive Waste Heat Recovery and Utilization
The electric drive system production process generates several valuable waste heat streams: heat rejection from air compressors, heat dissipated from product aging/burn-in lines in the main building, heat from test equipment in the R&D building, and heat from electrical room ventilation. The strategy is to capture and upgrade this heat for space heating and domestic hot water (DHW) preparation, displacing primary energy sources.
An economic analysis of different heat production methods establishes a priority order for waste heat use:
| Heat Source | Gas Consumption per kW (Nm³) | Electricity Consumption per kW (kWh) | Operating Cost per kW (CNY) |
|---|---|---|---|
| Gas-Fired Vacuum Boiler | 0.10 | 0.016 (aux.) | 0.413 |
| Air-Source Heat Pump (ASHP) | — | 0.50 (COP=2.0) | 0.425 |
| Air Compressor Waste Heat | — | 0.016 (pump only) | 0.014 |
| Aging Line Waste Heat via WHHP* | — | ~0.161 (COP~6.2) | 0.137 |
* Waste Heat Heat Pump
The analysis clearly shows that direct air compressor heat recovery is the most economical, followed by using high-temperature water-source heat pumps (WHHP) to harvest aging line heat. The use of primary gas boilers or ASHPs is less favorable from an operating cost perspective.
Detailed Waste Heat Utilization Schemes
1. Air Compressor Heat Recovery: The facility has multiple large air-cooled screw compressors. The recoverable heat can be estimated using a standard formula for air-cooled models:
$$Q_{rec} = n \times N \times \Phi \times k \times c$$
Where:
$n$ = number of compressors
$N$ = compressor motor power (kW)
$\Phi$ = heat recovery ratio (~70% for air-cooled)
$k$ = simultaneous use factor (~0.7)
$c$ = heat exchange efficiency (~0.85)
For this plant’s compressor fleet, the calculated maximum recoverable heat is approximately 341.5 kW at a temperature up to 70°C. This high-grade heat can be used directly via a plate heat exchanger to pre-heat DHW or space heating return water with minimal pumping energy.
2. Aging Line & Process Cooling Water Heat Recovery: The aging lines in the main production building and test equipment in the R&D building require year-round cooling. This results in a constant source of low-grade heat (e.g., 32°C return water). Instead of rejecting this heat to the atmosphere via cooling towers year-round, high-temperature water-to-water heat pumps (WHHPs) are employed. These WHHPs use the warm return water as their heat source to produce hot water at 50°C for space heating and DHW needs. The Coefficient of Performance (COP) for heating can exceed 6.0, making it extremely efficient.
3. Integrated System Design: The preliminary heating scheme relied on large gas-fired vacuum boilers for most buildings and ASHPs for the office. The optimized, integrated scheme uses a combination of:
– Smaller gas-fired boilers as peak/backup.
– A bank of high-temperature WHHPs (5 units) to recover aging line/R&D waste heat.
– Direct heat exchange from air compressor cooling circuits.
– A minimal number of ASHPs retained for the office building’s independent operation.
Seasonal heat balance diagrams were developed to size the components correctly. The integrated system maximizes the use of “free” waste heat before resorting to purchased gas.
Economic Analysis of Heating Schemes
| Scheme | Initial Investment* (10k CNY) | Avg. Operating Cost per kW (CNY) | Seasonal Operating Cost** (10k CNY) | Simple Payback vs. Prelim. |
|---|---|---|---|---|
| Preliminary Design (Boilers+ASHPs) | 321.61 | 0.415 | 709.65 | — |
| Integrated Waste Heat Scheme | 474.56 | 0.30 | 513.00 | ~2.33 months |
* Equipment cost only.
** Based on 90-day heating season, 20h/day average runtime.
The waste heat integration scheme increases investment by 1.5295 million CNY but reduces annual heating energy costs by 1.9665 million CNY, resulting in a payback period of about 2.33 months.
Condensate Water Recovery
An additional, often-overlooked saving measure is the recovery of air conditioning condensate. For every kW of cooling load, approximately 0.4–0.8 kg/h of condensate is generated. With a total cooling load of 19,800 kW, this yields 7.92–15.84 t/h of condensate at a temperature typically below 20°C. This water is of good quality and, when pumped to the cooling tower basins, serves as ideal makeup water. It reduces the load on the municipal water supply, lowers water costs, and because it is cooler than ambient temperature, it slightly improves the cooling tower’s thermal performance, thereby boosting chiller COP. The estimated seasonal savings are 75,700–151,400 CNY per cooling season, further enhancing the sustainability profile of the electric drive system plant.
Conclusion
The comprehensive redesign of the energy systems for this Hunan-based electric drive system factory demonstrates the profound impact of integrated engineering. By adopting a high-efficiency refrigeration plant philosophy—featuring magnetic bearing chillers, optimized hydronics, and smart zoning—the system EER was elevated from 4.16 to 5.56. Simultaneously, a systematic approach to waste heat recovery, prioritizing direct use and high-COP heat pumps, drastically reduced the demand for primary gas heating.
The economic analysis is compelling: the additional capital expenditure for both the high-efficiency cooling and waste heat recovery systems is recouped within 2–3 months of operation due to the dramatic reduction in energy costs. This case study provides a validated blueprint for achieving significant operational savings and carbon emission reductions in the manufacturing sector, particularly in energy-intensive industries like electric drive system production. It underscores that moving beyond standard design practices towards integrated, optimized energy systems is not only technically feasible but also immensely profitable.