Optimization of the Hybrid Energy Harvest Systems Sizing for Net-Zero Site-Energy Houses Keke Zheng 1 ; Yong K. Cho, A.M.ASCE 2 ; Ziqing Zhuang 3 ; and Haorong Li 4 Abstract: Energy-efficient and net-zero-energy buildings are quickly gaining importance and popularity among contemporary building prac- tices. Typically, several renewable energy–harvesting systems are combined to reach the objective of net-zero energy consumption, posing the unique challenge of properly sizing each energy system. Because no standard design procedure exists, installed energy systems often are inappropriately sized. In this paper, we propose a holistic design procedure to determine and optimize the corresponding size of renewable energy systems in a typical residential house to meet the total energy demand. To evaluate this procedure, a case study was implemented for the zero net energy test house at the University of Nebraska that uses three energy-source systems: geothermal, photovoltaic panel, and wind turbine. Although the results of the case study indicate that, based on present costs of renewable-energy sources, energy systems are more expensive to operate in a zero-energy house than in homes using grid power, the proposed size-optimization method for the renewable energy sources was found to be sound. DOI: 10.1061/(ASCE)AE.1943-5568.0000109. © 2013 American Society of Civil Engineers. CE Database subject headings: Renewable energy; Simulation; Optimization; Buildings; Hybrid methods; Energy efficiency. Author keywords: Renewable energy; Zero-energy house; Simulation; Optimization. Introduction In recent years, rising energy costs and looming climate concerns have caused net-zero-energy buildings to grow in importance and popularity. In a net-zero-energy house, energy needs are greatly reduced through efficiency gains so that the balance of energy needs can be supplied with renewable technologies (Torcellini et al. 2006). A zero-energy house can be defined in several ways depending on the boundary and the metric; four commonly used definitions are: net-zero site energy, net-zero source energy, net-zero energy costs, and net-zero energy emissions (Torcellini et al. 2006). Regional climatic conditions associated with a building, including orientation, solar irradiance, wind speed, and groundwater levels, all vary from location to location. To use energy effectively, specific climate con- ditions that may affect sources of energy should be considered in the design phase of a zero-energy building before physical design components are considered. Systems combining several energy sources are typically better at making maximum use of available energy sources than systems that rely on a single source of energy because of the possible instability of single-source energy pro- duction (Gevorkian 2010; Yang et al. 2007). The Hybrid Optimization Model for Electric Renewables (HOMER) energy-modeling software is currently the most widely used tool for designing and analyzing hybrid power systems. It per- forms the energy balance calculations for all system configurations that the designer wants to consider and then lists results for all possible system configurations. The optimal configuration can be determined by comparing each net present cost. HOMER is now widely accepted for designing large-scale village- or community-level electrical power systems. However, for a single off-grid house, the designed energy- supply system should provide enough real-time energy output to match the dynamic energy-demand requirement. In this situation, the annual energy match may not work for a single house. Also, HOMER’s optimization calculation does not integrate a thermal energy system, which is actually indispensable for using renewable energy and an important energy-consumption source. The optimization of renewable energy systems in which a battery bank is employed to implement size matching during the operating phase has been the focus of a number of studies that incorporate such methods as the least-squares method (Castle 1981; Elhadidy and Shaahid 1999), the tradeoff method (Bruke et al. 1988), and the loss- of-power-supply probability method (Abouzahr and Ramakumar 1991). Currently, the battery bank is the most expensive component in a typical energy system. An optimal energy system design ob- viously will reduce the capacity and size of the battery bank, as well as extend the replacement period. However, no related research has been reported about the optimization of hybrid integrated energy systems during the initial design period. This paper proposes a fundamental procedure, with an emphasis on the design phase, to optimize the design of the corresponding size for both the thermal and electrical energy systems. Methods Typical net-zero-energy houses integrate multiple energy sources to satisfy both the thermal load and electrical power requirements. Fig. 1 illustrates general energy flow for a typical off-grid net-zero site-energy residential home. 1 Research Engineering Associate, Energy Systems Laboratory, Texas A&M Univ., College Station, TX 77845. E-mail: kzheng@tamu.edu 2 Associate Professor, Charles Durham School of Architectural Engi- neering and Construction, Univ. of Nebraska–Lincoln, Omaha, NE 68182 (corresponding author). E-mail: ycho2@unl.edu 3 Graduate Research Assistant, Charles Durham School of Architectural Engineering and Construction, Univ. of Nebraska–Lincoln, Omaha, NE 68182. E-mail: zhuangziqing@gmail.com 4 Associate Professor, Charles Durham School of Architectural Engi- neering and Construction, Univ. of Nebraska–Lincoln, Omaha, NE 68182. E-mail: hli3@unl.edu Note. This manuscript was submitted on January 5, 2012; approved on August 29, 2012; published online on September 1, 2012. Discussion period open until February 1, 2014; separate discussions must be submitted for individual papers. This paper is part of the Journal of Architectural Engineering, Vol. 19, No. 3, September 1, 2013. ©ASCE, ISSN 1076- 0431/2013/3-174–178/$25.00. 174 / JOURNAL OF ARCHITECTURAL ENGINEERING © ASCE / SEPTEMBER 2013