Novel application of combined heat and power for multi-family residences and small remote communities

Date of Award

2017

Degree Name

Ph.D. in Mechanical Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Advisor: K. P. Hallinan

Abstract

Combined heat and power (CHP) systems are increasingly used in conjunction with traditional grid power for industrial and residential applications. This technology most often involves the on-site combustion of primary fuel, such that both electrical and thermal energy can be utilized to increase overall efficiency. It is also possible to create electrical and thermal energy from solar radiation, using hybrid photovoltaics and thermal (PVT) collectors. These are designed to lower the photovoltaic temperature, improving electrical efficiency, while providing useful thermal energy. One of the key steps in deploying CHP technology is optimal sizing and energy dispatch for a particular application. This work considers these problems for a natural gas powered CHP in a multi-family residential building in North America, and PVT for desalination in remote areas in the Kingdom of Saudi Arabia (KSA). It has already been established that CHP for building applications can reduce grid power requirement and lower overall energy costs. However, no comprehensive study has considered optimizing CHPs for multi-family residences. Although this type of building represents a significant fraction of overall energy consumption in the US and world, they have been shown to be significantly less efficient than other types of residences. Also, due to significant thermal demand in the form of hot-water, multi-family residences are particularly well-suited for CHP.Two separate natural gas powered CHP designs for a multi-family residence are presented in this work, both conceived as retrofits to an existing building. These designs use historical demand data from an all-electric 120-unit multi-family residence in Columbus, Ohio, US that was built in 2008 to minimum code standards. The first design uses a CHP that operates intermittently to meet partial loads for electricity and hot water in order to reduce overall energy cost, when considering a demand sensitive grid power cost pricing schedule. A mathematical model is developed for activating the CHP and dispatching its electric power to the building and thermal energy to a central hot water tank. The modeling includes a detailed cost function, which is optimized over the CHP and storage tank sizes under a constraint on the CHP duty cycle.The second CHP design for a multi-family residence considers a cold climate, such that the building would have greater thermal energy needs. In this case, the CHP is used in conjunction with a ground-coupled geothermal heat pump (GCHP) system, forming a hybrid design. GCHP systems use the ground as a heat source or sink to improve the efficiency of space heating and cooling, and GCHP is often used in residential and commercial buildings due to their higher efficiency and lower environmental impact. However, for a heating-dominated climate, the residential building would take more thermal energy from the ground in the winter than it returns in the summer, causing the ground temperature to drop over time. To correct this, the design presented here operates the CHP continuously, and passes its excess thermal energy to the ground, thus enabling the possibility for balancing the heating and cooling of the ground over each year. On the electrical side of the system, a battery storage element is added to better match the variations in load to the continuous CHP electrical output. The third CHP design considered in this work uses PVT for desalination in a hot, dry climate. As global demand for fresh water increases, desalination technology is becoming more important because natural supplies of fresh water are fixed. Desalination activity is largely concentrated in the Middle East, where dry Arab countries rely on desalination to meet their fresh water demand. The energy needed for desalination in the Middle East is mainly provided by burning oil, raising concerns about greenhouse gas (GHG) emissions and, frankly, increasingly depleted supply. In this context, this work presents a PVT design to power reverse-osmosis membrane desalination, most appropriate for small, remote communities in KSA. It has been shown that the energy demands for RO can be reduced by pre-heating the feed brine. Therefore, the design uses the thermal energy from PVT to pre-heat the feedwater and the electrical energy to satisfy the RO pumping demands. Thermal and battery storage, along with conventional backup power, are necessary in order to operate the RO continuously and utilize all of the renewable energy collected by the PVT. The design allows for sizing of the components in order to achieve minimum cost at any desired level of renewable energy penetration. The performance of each design presented in this work is measured primarily in terms of economic cost and carbon reduction. Savings relative to using conventional grid power are computed, allowing for determination of payback time and net present value. Results indicate that each CHP design provides both cost advantage and carbon reduction, spread out over the system lifetime. The scale of the advantages is examined as a function of parameters such as natural gas and grid power prices.

Keywords

Cogeneration of electric power and heat Testing, Cogeneration of electric power and heat Cost effectiveness, Hybrid power Testing, Hybrid power Cost effectiveness, Mechanical Engineering, Combined heat and power, Hybrid Combined heat and power Geothermal System, PVT System with Desalination, Cost Optimal Combined heat and power for a MultiFamily Building

Rights Statement

Copyright © 2017, author

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