An Overall Assessment of Ice Storage Systems for Residential Buildings

The recent availability of variable electric energy and demand rates for residential buildings is providing incentives for the application of thermal storage for cooling that previously has been limited to commercial buildings. This is particularly relevant for hot climates where air-conditioning (A/C) use is the primary cause for peak electricity demand. Thermal storage allows consumers to store “cooling” when demand is low and minimize operation of the A/C during peak periods. From an economic perspective, the use of storage can significantly reduce operating costs depending on the utility rate incentives. In addition, storage can lead to a reduction in the installed cost of the primary cooling equipment because of a reduction in the peak equipment cooling requirement. However, this reduced equipment cost is counteracted by the additional costs required for storage and a secondary loop. This paper considers the overall economics associated with a packaged A/C integrated with ice energy storage for residential cooling applications. The evaluation was performed using a model of the proposed system that estimates system performance and operating cost over a cooling season for different locations and utility rates and using a generalized control strategy presented in a companion paper. The proposed system is compared to a conventional spilt system A/C in terms of initial cost, operating cost, and economic payback. In addition, we investigate the trade-off between equipment cooling capacity and storage size to determine minimum payback period for each situation. The optimization results show that the systems with the shortest payback period have initial costs that are very similar to the baseline system. In addition, the payback periods are attractive in locations with favorable utility rates and long cooling seasons (i.e., hot climates)

Development of a Generalized Control Strategy for Thermal Energy Storage in Residential Buildings

In recent years, variable electricity pricing has become available to residential consumers to incentivize demand reductions during traditional midday peak hours. This is especially important in hot climates where air-conditioning (A/C) use is the primary cause for peak electricity demand. Thermal storage allows consumers to store “cooling” when demand is low and minimize operation of the A/C during peak periods. This paper considers a packaged A/C integrated with thermal energy storage using ice for residential cooling applications. The focus of the paper is the development and validation of a generalized control strategy that can be used for available residential utility rate structures that include different combinations of time-of-use energy and demand charges. The generalized control strategy is based on a unique combination of different heuristic strategies for charging and discharging of storage that are typically applied to commercial-scale A/C systems with integrated thermal energy storage. In order to evaluate overall performance, a model of the proposed system is developed and used to calculate cooling season operating costs for different geographic locations and utility rates. The performance of the generalized strategy is evaluated in comparison to the most commonly employed control strategy for commercial ice storage systems, called chiller priority control. A range of unit capacities, storage sizes, geographic locations, and residential utility rates are considered. The resulting decrease in operating cost with the generalized control strategy, when compared to chiller priority control, was as much as 50% based on the utility rates considered in this paper

CFD Simulations of Single- and Twin-Screw Machines with OpenFOAM

Over the last decade, Computational Fluid Dynamics (CFD) has been increasingly applied for the design and analysis of positive displacement machines employed in vapor compression and power generation applications. Particularly, single-screw and twin-screw machines have received attention from the researchers, leading to the development and application of increasingly efficient techniques for their numerical simulation. Modeling the operation of such machines including the dynamics of the compression (or expansion) process and the deforming working chambers is particularly challenging. The relative motion of the rotors and the variation of the gaps during machine operation are a few of the major numerical challenges towards the implementation of reliable CFD models. Moreover, evaluating the thermophysical properties of real gases represents an additional challenge to be addressed. Special care must be given to defining equation of states or generating tables and computing the thermodynamic properties. Among several CFD suite available, the open-source OpenFOAM tool OpenFOAM, is regarded as a reliable and accurate software for carrying out CFD analyses. In this paper, the dynamic meshing techniques available within the software as well as new libraries implemented for expanding the functionalities of the software are presented. The simulation of both a single-screw and a twin-screw machine is described and results are discussed. Specifically, for the single-screw expander case, the geometry will be released as open-access for the entire community. Besides, the real gas modeling possibilities implemented in the software will be described and the CoolProp thermophysical library integration will be presented