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Equivalent Energy Storage Capability of Combined Heat Pump and Biogas Engine Generator System

Variable renewable energy generations (RE) such as photovoltaic generation systems and wind power generation systems are being developed rapidly in the world due to their environmental advantages. On the other hand, RE outputs are variable and intermittent depending on the weather condition. For the supply and demand balancing control in a whole power system, introduction and application of energy storage systems (ESS), such as battery energy storage systems (BESS), are actively discussed in Japan. Usage of BESS is technically feasible, but still difficult from economical viewpoint due to their expensive installation cost and round-trip conversion losses. New energy and industrial technology development organization (NEDO) in Japan is promoting “R&D Project for Integration of Variable Renewable Energy: Mitigation Technologies on Output Fluctuations in Power Grid Systems” in FY2014-2018. One of the missions of this project is a feasibility study on alternative ESS, which can be used for mitigating the RE output fluctuation. For this purpose, the authors developed a combined heat pump and biogas engine generator system (CHBS) which has the equivalently same function as ESS. CHBS can control the direction and amount of electricity output flexibly while supplying constant thermal output by changing the heat supply ratio between heat pumps (HP) and biogas engine generators (BG). More specifically, apparent charging function is realized by using the HP, which converts electricity into heat, and apparent discharging function is realized by using the BG, which co-generates electricity and heat together. The largest advantage of CHBS is its easy-implementation; only an additional HP installation into an existing biogas plant with BG can provide equivalent ESS function.
This paper proposes and evaluates the equivalent energy storage capability in both theoretical and empirical approaches. The equivalent energy storage capability is measured in terms of kW and kWh capacities, which are widely used for a typical BESS. Therefore, direct quantitative comparisons of CHBS and BESS become able by the proposed equivalent capability.
For the theoretical equivalent capability, electricity and heat energy flows in CHBS is mathematically formulated. Based on the max-flow and the persistence duration analyses, the equivalent kW and kWh capacities are defined. Relationship between the operation condition of CHBS and the resultant equivalent capability is mathematically and graphically shown in the full paper.
To evaluate the empirical equivalent capability, this paper proposes control strategies for CHBS to mitigate the RE output fluctuation. The proposed methods consist of two steps. In the first step, optimal operations of HP and BG are determined in advance for the forecasted RE outputs data. In the second step, the operations of HP and BG are modified based on the monitored RE outputs data. This paper also evaluates the empirical equivalent capability of CHBS for the actual RE forecast and output data through computational simulations in MATLAN/Simulink environment.

Author(s):

Ryoichi Hara    
Hokkaido University
Japan

Hiroyuki Kita    
Hokkaido University
Japan

Shiho Ishikawa    
Hokkaido University
Japan

 

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