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While buildings served by multiple rooftop units are widely prevalent in retail and food industries, optimal coordination among rooftop units has received little attention. Currently, most buildings utilize plain feedback or heuristic strategies to determine rooftop unit operations. This article presents a switched dynamic programming based approach for optimal scheduling of rooftop unit operations. Using a mid-sized restaurant served by multiple rooftop units as an example, a switched affine dynamic system model is first presented. The rooftop unit coordination control problem is then formulated as a switched optimal control problem with nontrivial switching cost. A dynamic programming based method is proposed for the solution of the switched optimal control problem, which has reasonable computational complexity and can be implemented either as a one-shot solution or as part of a model predictive control algorithm. To mitigate the growing computational complexity of the proposed algorithm, the approach is extended by proposing a distributed extension of the model predictive control formulation. Finally, a simulation based evaluation of the various formulations is conducted, and the salient features of the formulation are highlighted. [ABSTRACT FROM PUBLISHER]

Abstract: A detailed mechanistic model has been developed for scroll compressors and scroll expanders with liquid flooding for application to the Liquid Flooded Ericsson Cycle. This model is based on the integration of a set of differential equations that arise from the conservation laws. The impact of the flooding liquid on the working process is also included through mixture properties as well as treatment of two-phase flow pressure drop. With the working process known, it is then possible to solve for the compressor power, total mass flow rate and other model outputs. The fine level of detail in the model allows for the investigation of the impact of design changes to the scroll machines. This model can also be readily extended to allow for the analysis of liquid flooded vapor compression refrigeration. [Copyright &y& Elsevier]

Abstract: The use of liquid-flooding in the compression and expansion of non-condensable gas in scroll compressors and expanders enables the possibility of quasi-isothermal working processes. Liquid-flooded scroll machines were installed in a fully-instrumented Liquid-Flooded Ericsson Cycle test rig to conduct entire cycle performance tests. In addition, detailed compressor and expander performance data was obtained. Oil mass fractions of up to 92% and 76% were added to the gas entering the scroll compressor and expander respectively. The overall isentropic efficiency of the scroll compressor based on the shaft power with flooding was up to 73% and the volumetric efficiency was above 92%. For the expander, the best overall isentropic and volumetric efficiencies achieved were 66% and 105% respectively. The mechanistic model presented in the companion paper was validated against the experimental data for both the compressor and the scroll expander with good agreement, though the agreement is better for the scroll compressor. [Copyright &y& Elsevier]

Abstract: The objective of this study was to measure the effects of improper airflow and refrigerant charge on the seasonal performance of a typical 10.6kW, R-410A residential heat pump with a thermostatic expansion valve. Heating and cooling tests were performed in combinations of three refrigerant charges of 75%, 100%, and 125% of nominal value and two airflows of 75% and 100% of rated airflow. In addition, cyclic tests were performed to estimate the heating and cooling seasonal coefficient of performance (COP) at six climate zones specified by Air-Conditioning, Heating, and Refrigeration Institute (AHRI) Standard 210/240-2008. Results showed that, in each climate zone, increases in refrigerant charge at the rated airflow could improve the unit''s heating seasonal COP by as much as 5%. However, combined decreases in airflow and refrigerant charge could penalize the unit''s heating seasonal COP by as much as 10%. [Copyright &y& Elsevier]

There are numerous possible technology combinations for providing desalination using renewable energy sources. This paper presents a model that captures facets ranging from hourly availability of the renewable energy sources to economic parameters such as inflation and discount rate without relying on third-party software. The proposed approach considers different combinations of reverse osmosis and multi-stage-flash distillation driven by wind, solar PV and solar thermal with reference systems driven from the electric grid or natural gas. The levelized cost of water and the primary energy usage are presented as the main indicators for the performance of the different system architectures. Aruba is chosen as an example to show an application of the model. The project duration and the efficiency of desalination technologies is used to demonstrate the model capability for sensitivity analyses. The authors provide all used equations in the supplemental material to this paper. • Modeling desalination systems including the desalination technologies SWRO and MSF and the energy sources wind, solar PV, solar thermal, electric grid and natural gas • Complete model without need for third party software • All equations in supplemental material [ABSTRACT FROM AUTHOR]

• Experiments were conducted to optimize the control of TES defrosting for CASHPs. • To regulate heat distribution, the LSC and HSC refrigerant flow rates were adjusted. • Optimization included changing the LSC compressor frequency and HSC EEV opening. • LSC compressor frequency of 90 Hz is recommended. • Reducing the HSC EEV opening is recommended. Thermal energy storage based (TES-based) reverse cycle defrosting method is a feasible way to reduce energy requirements for defrosting of cascade air source heat pumps (CASHPs). The energy stored in the phase change material based heat exchanger (PCM-HE) is the heat source for both the high stage cycle (HSC) and low stage cycle (LSC) during the TES-based reverse cycle defrosting process, thus the heat provided to the HSC and LSC are coupled, affecting the defrosting performance in LSC and heating performance in HSC. To shorten the defrosting time and maintain the indoor thermal environment, the heat distribution between HSC and LSC during TES-based reverse cycle defrosting was explored. It was found that the heat distribution pattern was mainly influenced by the refrigerant flowrate. Therefore, controlling the refrigerant flowrate of HSC and LSC during the TES-based reverse cycle defrosting is a possible way to regulate the heat distribution between HSC and LSC. In this paper, an experimental investigation of the control strategy for TES-based reverse cycle defrosting of CASHPs is presented. The defrosting performance and indoor heating performance during TES-based defrost operation with LSC compressor frequencies of 60 Hz, 75 Hz, 90 Hz, and 105 Hz were tested. Then HSC EEV openings of 80%, 90% and 100% at the optimized LSC compressor frequency of 90 Hz were considered. It was concluded that the optimal TES-based reverse cycle defrosting strategy is to maintain the LSC compressor frequency at 90 Hz and the HSC EEV opening at 80% for the equipment considered in this study. The defrosting time was 440 s and the average indoor heating capacity was 5.23 kW. The heat provided to the HSC and LSC accounted for 38.7% and 61.3% of the total heat released by PCM-HE, respectively. [ABSTRACT FROM AUTHOR]