Smart Operational Control Strategies for Gas-Fired CCHP Distributed Energy Systems

Smart Operational Control Strategies for Gas-Fired CCHP Distributed Energy Systems

Gas-centered combined cooling, heating, and power (CCHP) distributed energy systems are currently the mainstream form of distributed energy. Their operating efficiency directly determines project profitability and ultimate success. Intelligent energy operation and management systems based on the Industrial Internet of Things (IIoT), data science, and industrial artificial intelligence can effectively improve energy coupling, thereby enhancing the intelligence level and operating efficiency of gas-fired CCHP distributed energy systems. This article introduces the control strategies and principles involved in the intelligent operation of gas-fired CCHP systems. The same concepts can also be applied to integrated distributed energy systems such as biomass cogeneration, waste-to-energy plants, and heat pump systems.

1. Introduction to Control Strategies

A gas-fired CCHP distributed energy system uses natural gas as its primary energy input to meet the energy demands of individual buildings or building clusters. Compared with large centralized energy supply systems such as utility-scale power plants and district heating centers, it offers advantages including higher energy utilization efficiency, better energy quality, energy savings, emissions reduction, and improved reliability. In project implementation, gas-fired CCHP systems often need to be combined with technologies such as ground-source heat pumps, conventional water-source heat pumps, ice thermal storage, heat storage, and solar thermal utilization, while also considering the coordinated use of a certain amount of conventional energy. This results in a highly integrated energy conversion and supply system.

Whether a gas-fired CCHP distributed energy system can achieve its process design goals depends on selecting and implementing appropriate design, construction, operation, and maintenance plans based on project characteristics, along with suitable control strategies and control system hardware. Reasonable control strategies and control hardware therefore form an essential part of the control system for gas-fired CCHP distributed energy systems. The control system serves all disciplines or subsystems within the energy station, including thermal process engineering, electrical systems, HVAC, gas systems, fire alarm systems, water treatment systems, conveying systems, and more. The control strategy is the set of control logic requirements that these disciplines and systems impose on the control system.

The control strategies of gas-fired CCHP distributed energy systems arise from engineers’ pursuit of overall and subsystem-level targets such as safety, stability, high efficiency, energy conservation, cost reduction, and emissions reduction. In essence, a control strategy is the method by which technical personnel seek to achieve these goals as comprehensively as possible, while balancing and optimizing among them.

More specifically, control strategies should focus on ensuring supply, safe operation, energy conservation and consumption reduction, economic operation, and integrated optimal dispatch, so as to maximize the advantages of gas-fired CCHP distributed energy systems.

2. Basic Operating Principles

The following are the basic operating control principles for gas-fired CCHP distributed energy systems.

(1) Improve overall energy utilization efficiency

For gas-fired CCHP distributed energy systems with installed capacity less than or equal to 15 MW, the Technical Code for Gas-Fired Cooling, Heating, and Power Distributed Energy Engineering clearly requires that the annual average comprehensive energy utilization efficiency of the distributed energy system must exceed 70%. This mandatory requirement is a key benchmark for verifying the high-efficiency gas utilization characteristics of gas-fired CCHP technology and is also a guarantee of its economic and social benefits. For distributed energy systems above 15 MW, relevant standards should likewise be referenced to improve overall energy utilization efficiency.

(2) Ensure a high number of full-load operating hours

When generator capacity is fixed, it is necessary to reasonably adjust load distribution among generator sets to maintain high operating efficiency and achieve a higher annual number of full-load operating hours. Generator investment accounts for a relatively large share of the total investment in an energy station, and its annual operating time directly affects investment returns. A higher annual full-load hour count improves the techno-economic performance of the generator investment, thereby enhancing the project’s overall economics.

(3) Achieve cascaded utilization of waste heat

The operation of gas-fired CCHP distributed energy systems should maximize the use of recovered waste heat from power generation and reduce the discharge of higher-grade energy such as exhaust gas heat and jacket water heat. In particular, when there is a conflict between operational economics and complete waste heat utilization, priority should be given from the perspective of resource conservation to fully utilizing waste heat and minimizing waste.

(4) Maximize power generation efficiency

Although the electrical generation efficiency of a generator generally cannot be fundamentally changed, the generator’s operating condition can be adjusted so that it runs in a higher-efficiency range. Small gas-fired internal combustion generators typically have electrical efficiencies of 35% to 45%, which is lower than the approximately 50% efficiency of large gas-fired power plants. If gas generator sets cannot operate at relatively high efficiency, then from a pure power supply perspective, distributed energy systems are less advantageous than large gas-fired plants. However, from the perspective of meeting multiple energy demands simultaneously and delivering energy over short distances, gas-fired CCHP distributed energy systems are superior to large centralized gas power plants. Therefore, electrical generation efficiency in distributed energy systems should be improved as much as possible to narrow the gap with large power plants while enhancing overall system efficiency and power quality.

(5) Rational use of peak-shaving equipment

Gas-fired CCHP distributed energy systems should be able to respond to changes in user loads. While ensuring full utilization of power generation equipment and associated waste heat recovery equipment, they should select peak-shaving equipment and peak-shaving methods rationally according to factors such as energy prices and load conditions, so as to achieve the lowest-cost peak-shaving mode and improve project economics.

3. Operating Strategies for Regional Projects

Regional projects generally have large installed generating capacity, a wide energy supply area, and a building service area exceeding one million square meters. Given a defined cooling and heating service area, different types of loads within the region have different characteristics. The operating strategy should take into account the load patterns at different times throughout the year and formulate targeted operating plans. For regional projects, operating strategies should particularly consider the following points:

1. Daily operating strategies should fully account for the fluctuation characteristics of cooling loads. For centrally cooled public buildings, cooling loads vary significantly with building occupancy and schedules, typically peaking around 4:00 p.m., while nighttime loads are relatively low.

2. For northern projects requiring district heating, the total regional heating load during winter daytime does not fluctuate greatly. System operation can therefore refer to conventional CHP heating system practices, while still incorporating energy-saving measures such as climate compensation.

3. For buildings with distinct functions, such as office buildings and shopping malls, regional electricity demand changes significantly with occupancy schedules. In summer, electricity use reaches the annual peak, and demand remains high from 12:00 to 16:00. Residential electricity usually accounts for a relatively small share. Although residential consumption rises at night, total nighttime power demand is still relatively low. Regional projects generally export electricity to the grid, and output is subject to dispatch by local power authorities, so regional electrical load is not fully correlated with generation output. However, for islanded projects, electrical load characteristics must be considered when dispatching generator sets.

4. In regional projects, the duration of peak cooling load operation and annual full-load cooling hours is relatively short. Cooling demand fluctuates significantly throughout the cooling season. In general, peak cooling load is high, but high load factors occur only for limited periods, and the load duration curve is relatively steep. During the cooling season, only part of the time does cooling demand exceed 50% of the design load; for the rest of the time, it remains below 50%. Therefore, for projects requiring summer cooling, system operating strategies under different load conditions should be considered.

5. For projects whose generating capacity is designed based on the region’s base electrical load, special attention should be paid to operating modes during the heating season and shoulder seasons when electricity demand is relatively low. In addition, strategies should be developed for situations in which electrical load does not match cooling and heating loads.

4. Operating Strategies for Building Cluster Projects

This type of project generally supplies energy to several or multiple buildings. Installed capacity is medium-sized, and the serviced building area is usually between 200,000 and 1,000,000 square meters. Project load characteristics fall between those of large regional projects and single-building projects and are heavily influenced by the functions of the individual buildings. If accurate hourly load calculation results for each building over the entire year can be obtained, reasonable operating strategies should be developed based on those results.

1. For projects composed of multiple buildings with different functions, there may be balancing effects among various aggregate loads. The specific load patterns of the project should therefore be analyzed and summarized, and an appropriate operating plan should be developed through careful operational analysis.

2. For projects consisting of multiple buildings with the same or similar functions, a certain amount of energy storage capacity should be considered. Based on reasonable load forecasting, the storage capacity and suitable discharge timing should be determined. For projects without time-of-use electricity pricing, a techno-economic analysis should be conducted to balance reduced main equipment investment against increased energy storage system investment.

3. This type of project often requires gas-fired CCHP distributed energy technology to be integrated with other energy technologies, forming an energy system in which distributed energy plays the primary role and other technologies play a supporting role. Under different external energy prices and unit efficiency conditions, different supply modes lead to different costs. Therefore, if a project adopts a configuration with multiple energy supply modes, economically reasonable operating strategies should be formulated according to external conditions and unit status at different times.

5. Operating Strategies for Building-Scale Projects

Building-scale projects generally serve a single building or a small number of buildings. Installed generator capacity is relatively small, and the serviced building area is typically under 200,000 square meters. The load characteristics of building-scale projects depend entirely on the functional type of the building. Professional load calculation tools should be used to perform hourly annual load calculations for the building, and reasonable system operating strategies should then be developed on that basis.

For building-scale projects, installed capacity is usually small, and the number of major pieces of equipment involved is limited, typically no more than eight units. As a result, the system itself usually does not have many operating mode variations. Therefore, the operating strategy for building-scale projects mainly follows the basic operating principles of gas-fired CCHP distributed energy systems, while also considering flexible switching control methods for generators and the building power distribution system, so as to achieve higher electrical efficiency and greater annual power generation.

6. Optimization of Operating Strategies

On the premise of establishing the above operating strategies, analysis of operational data across different operating periods can yield a series of performance indicators. The most important of these include the total energy utilization efficiency of the gas-fired CCHP distributed energy system, total energy consumption, total energy use, and total energy supply.

The operating conditions of a gas-fired CCHP distributed energy system change constantly from moment to moment. Various system loads also change with external conditions, and the status of each unit changes accordingly.

To use historical data analysis to guide system operation, optimize operation according to load changes, and integrate operational experience and control strategies, it is necessary to establish a higher-level optimization and analysis system. This optimization and analysis system may be part of the control system itself, or it may be independent from the control system, but in either case it should be able to perform operating strategy optimization and analysis.

Optimization of operating strategies should be based on theoretical calculations, analysis of actual operating data, and various kinds of operating experience. According to the requirements of project management, the corresponding optimization direction should be selected and different optimized operating strategies should be implemented.