Development of advanced control technologies for building HVAC systems to provide frequency regulation service to smart power grids

Abstract

The instantaneous balance and reliability of power grids (reflected in the power grid frequency) is conventionally guaranteed through frequency regulation provided at the supply side. However, more frequency regulation capacity will be needed due to the increasing involvement of intermittent renewable power generations. On the other hand, with the assistance of smart grid technologies, such as communication systems, smart meters and smart energy management systems, demand resources are enabled to take part in the power market and provide frequency regulation service. To provide this service, demand resources should follow the frequency regulation signal given by power grids which represents the magnitude of power imbalance between supply side and demand side. In this way, they can help the power grids to relieve the pressure of power balance and maintain the frequency of power grids within an acceptable range. Heating, ventilation, and air-conditioning (HVAC) systems in buildings, at the demand side, are promising candidates for providing frequency regulation service. It is because they account for a large proportion of electric energy consumption and have great power use flexibility. However, there are many problems and challenges in practice. From the viewpoint of power grids, the main problems and challenges include: 1) the quality and 2) the capacity of frequency regulation service provided by HVAC systems. From the viewpoint of buildings at demand side, the main problems and challenges are: 3) the impact of providing frequency regulation service on indoor environment control and 4) the impact of providing frequency regulation service on the efficiency of HVAC systems. These four problems/challenges are not well addressed in existing studies. For some problems/challenges, existing studies have drawn contradictory conclusions. This PhD study, therefore, attempts to comprehensively and systematically study the use of HVAC systems for providing frequency regulation service and to develop advanced control technologies to address the problems concerned. The 1st and the 3rd problems/challenges are first investigated by experimental study. In particular, a variable-speed pump in an HVAC system is used to provide frequency regulation service in the experimental study. A control strategy for variable-speed pumps in providing frequency regulation service is proposed and implemented on a test platform. The experimental results show that variable-speed pumps could provide high-quality frequency regulation service (the 1st problem/challenge). For the 3rd problem/challenge, the experimental results indicate that the impact of providing frequency regulation service on indoor temperature is not very significant when following the test signals. However, the impact on supply air temperature is significant, and this impact increases when the frequency of the frequency regulation signal increases. In the test, the fluctuation magnitude of the supply air temperature increases from 3.30 K to 7.10 K when the frequency of the frequency regulation signal increases. Such large fluctuation could deteriorate the indoor environment control when the air distribution in an indoor space and unbalanced cooling/heating distribution among indoor spaces are considered. A disturbance compensation enhanced control strategy is then proposed to prevent this sacrifice of the building indoor environment control quality (the 3rd problem/challenge) when providing frequency regulation service. The core element of this control strategy is a frequency disturbance compensation scheme, developed based on the concept of "disturbance-observer-based control". Experimental results show that the use of the proposed strategy can achieve significant improvement in building indoor environment control without sacrificing the quality of frequency regulation service. The fluctuation magnitude of supply air temperature to the indoor space was reduced from the range between 5.00 K and 7.61 K to the range between 2.40 K and 2.79 K when following two frequency regulation test signals. A further study on the impact of the service on indoor environment control (the 3rd problem/challenge) is conducted considering practical frequency regulation signals besides the dedicated test signals provided by power grids for assessing the quality of frequency regulation service. A set of criteria is proposed to assess the demanding level of frequency regulation signals to buildings. The impacts of providing frequency regulation service on indoor environment control are quantified when HVAC systems are following practical signals with different demanding levels to buildings. The results show that indoor temperature can have a relatively large offset when HVAC systems follow frequency regulation signals demanding to buildings. In addition, the indoor temperature offset increases when regulation capacity provided increases. Two dedicated test signals for buildings are recommended to verify the environment control performance of buildings when providing frequency regulation service to power grids. The impact of service provision on the efficiency of HVAC systems (the 4th problem/challenge) is analyzed and discussed to explain the contradictory conclusions in existing studies. A systematic analysis of the "round-trip efficiency" (used to assess efficiency) of building HVAC systems under various scenarios is conducted, based on the basic thermodynamic laws and fundamental energy system operation mechanisms/dynamics. Results prove that the conflicts among previous studies are caused by: 1) the fact that each of the previous studies reflected parts of the possibilities; and 2) the difference of assessment methods. Then the assessment methods are further analyzed by elaborating two hypothetical cases. A series of prerequisites are recommended for assessing the impact of service provision on the efficiency of HVAC systems. At last, a regulation bidding and operation control strategy is proposed. It can automatically optimize regulation capacity (the 2nd problem/challenge) and can control HVAC systems to provide qualified frequency regulation service, considering the tradeoff between financial reward (regulation capacity) and thermal comfort while satisfying the operating constraints of HVAC systems. The proposed control strategy is tested and validated on a simulation test platform. Results show that the strategy can maximize the use of regulation capacity provided by HVAC systems while ensuring the indoor environment control quality under a given guarantee rate. To conclude, comprehensive studies are conducted, and advanced control technologies are developed to address the problems/challenges encountered when adopting building HVAC systems for providing frequency regulation service. These problems/challenges include the quality and capacity of the service and the impacts of service provision on indoor environment control and the efficiency of HVAC systems.