Abdolreza Zaltash

Evaluation of Different Efficiency Concepts of an Integrated Energy System (IES)

The Integrated Energy System (IES) market in the United States (US) and worldwide has been increasingly expanding over the last few years. But there is still a lot of disagreement in interpretation of one of the most important IES performance parameters – efficiency. Some organizations, for example, use higher heating value (HHV) of fuel in efficiency calculations while some use lower heating value (LHV). Some accounts for auxiliary and parasitic losses while others do not. Some adhere to the “first-law” of efficiency while some use other methods, i.e., calculations recommended by the Federal Energy Regulatory Commission or the US Combined Heat & Power Association. Different efficiency concepts based on actual performance testing from the IES Laboratory at Oak Ridge National Laboratory (ORNL) are evaluated in this paper. The equipment studied included: a 30-kW microturbine, an air-towater heat recovery unit (HRU), a 10-ton (35 kW) hot waterfired (indirect-fired) single-effect absorption chiller, and a direct-fired desiccant dehumidification unit. Efficiencies of different configurations of the above-mentioned equipment based on various approaches are compared. In addition, IES efficiency gains due to the replacement of a 1 st generation HRU (effectiveness of approximately 75%) with a 2 nd generation HRU (effectiveness of approximately 92%) for the same IES arrangement are discussed. The results showed that the difference in HHV- and LHV-based efficiencies for different IES arrangements could reach 5-8%, and that the difference in efficiency values calculated with different methods for the same arrangement could reach 27%. Therefore, it is very important to develop standard guidelines for efficiency calculations that would be acceptable and used by the majority of IES manufacturers and end-users. At the very least, every manufacturer or user should clearly indicate the basis for their efficiency calculations.

Performance Evaluation of a 4.5 kW (1.3 Refrigeration Tons) Air-Cooled Lithium Bromide/Water Hot-Water-Fired Absorption Unit

During the summer months, air-conditioning (cooling) is the single largest use of electricity in both residential and commercial buildings with the major impact on peak electric demand. Improved air-conditioning technology has by far the greatest potential impact on the electric industry compared to any other technology that uses electricity. Thermally activated absorption air-conditioning (absorption chillers) can provide overall peak load reduction and electric grid relief for summer peak demand. This paper describes an innovative absorption technology based on integrated rotating heat exchangers to enhance heat and mass transfer resulting in a potential reduction of size, cost, and weight of the “next generation” absorption units. This absorption chiller (RAC) is a 4.5 kW (1.3 refrigeration tons or RT) air-cooled lithium bromide (LiBr)/water unit powered by hot water generated using the solar energy and/or waste heat. Typically LiBr/water absorption chillers are water-cooled units which use a cooling tower to reject heat. Cooling towers require a large amount of space and increase start-up and maintenance costs. However, RAC is an air-cooled absorption chiller which requires no cooling tower. The purpose of this evaluation is to verify RAC performance by comparing the Coefficient of Performance (COP or ratio of cooling capacity to thermal energy input) and the cooling capacity results with those of the manufacturer. The performance of the RAC was tested at Oak Ridge National Laboratory (ORNL) in a controlled environment at various hot and chilled water flow rates, air handler flow rates, and ambient temperatures. Temperature probes, mass flow meters, rotational speed measuring device, pressure transducers, and a web camera mounted inside the unit were used to monitor the RAC via a web control-based data acquisition system using Automated Logic Controller (ALC). Results showed a COP and cooling capacity of approximately 0.58 and 3.7 kW respectively at 35°C (95°F) design condition for ambient temperature with 40°C (104°F) cooling water temperature. This is in close agreement with the manufacturer data of 0.60 for COP and 3.9 kW for cooling capacity. Future work will use these performance results to evaluate the potential benefits of rotating heat exchangers in making the “next-generation” absorption chillers more compact and cost effective without any significant degradation in the performance. Future studies will also evaluate the feasibility of using rotating heat exchangers in other applications.

PERFORMANCE ANALYSIS OF INTEGRATED ACTIVE DESICCANT ROOFTOP AIR- CONDITIONING SYSTEM OPERATING IN HEATING MODE

This investigation describes the performance study of a novel Integrated Active Desiccant-Vapor Compression Hybrid Rooftop (IADR) at Oak Ridge National Laboratory (ORNL) in heating mode. The tests were performed at two different ratios of outdoor/return air. Analysis of performance characteristics under each operating mode, including heating capacity and energy efficiency ratio, are given. Results of defrost cycle are also presented. Comparison between the experimental performance of IADR unit and the calculated performance of other commercially available heat pump systems at comparable operating conditions has been conducted.Copyright

Frost Growth CFD Model of an Integrated Active Desiccant Rooftop Unit

A frost growth model is incorporated into a Computational Fluid Dynamics (CFD) simulation of a heat pump by means of a user-defined function in a commercial CFD code. The transient model is applied to the outdoor section of an Integrated Active Desiccant Rooftop (IADR) unit in heating mode. IADR is a hybrid vapor compression and active desiccant unit capable of handling 100% outdoor air (dedicated outdoor air system) or as a total conditioning system, handling both outdoor air and space cooling or heating loads. The predicted increase in flow resistance and loss in heat transfer capacity due to frost build-up are compared to experimental pressure drop readings and thermal imaging. The purpose of this work is to develop a CFD model that is capable of predicting frost growth, a potentially valuable tool in evaluating the effectiveness of defrost-on-demand cycles.

Baseline, Exhaust-Fired, and Combined Operation of Desiccant Dehumidification Unit

The performance of a commercially available direct-fired desiccant dehumidification unit (DFDD) has been studied as part of a microturbine generator (MTG)-based Integrated Energy System (IES) at Oak Ridge National Laboratory (ORNL). The IES includes a second-generation air-to-water heat recovery unit (HRU) for the MTG. The focus of these tests was to study the performance of a DFDD in baseline (direct-fired with its natural gas burner) mode and to compare it with a DFDD performance in the exhaust-fired and combined modes as part of the ORNL IES, when waste heat received from the MTG was used for desiccant regeneration. The baseline tests were performed with regeneration air heated by a natural gas burner (direct-fired). The testing of the waste-heat, or exhaust-fired DFDD as part of IES involved using the exhaust gas from the HRU for regeneration air in the DFDD after hot water production in the HRU. Hot water from the HRU was used to produce chilled water in an indirect-fired (water fired) absorption chiller. The combined DFDD was the combination of natural gas burner and exhaust-fired testing. The study investigated the impact of varying the process and regeneration conditions on the latent capacity (LC) and latent coefficient of performance (LCOP) of the DFDD, as well as overall IES efficiency. The performance tests show that LC increases with increasing dew point (humidity ratio) of the process air or the increased amount of waste heat associated with increased MTG power output. In addition, baseline LC was found to be three times higher than the LC in the exhaust-fired mode of operation. LCOP in baseline operation is also almost three times higher than that obtained in the exhaust-fired mode (55.4% compared to 19%). But, at the same time, addition of the DFDD to the IES with the MTG at maximum power output increases the overall IES efficiency by 4–5%. Results of the combined tests performed at a reduced MTG power output of 15 kW (51,182 Btu/h) and their comparison with the baseline and exhaust-fired tests show that activation of the DFDD gas burner during exhaust-fired tests increases the LC over the baseline value from 91,514.9 Btu/h (25.8 kW) to 101,835.8 Btu/h (29.8 kW). The LCOP during the combined mode is less than the “baseline” LCOP, because in addition to gas input, the low-grade MTG/HRU exhaust heat input to the DFDD are also being considered. The overall IES efficiency during the combined mode is approximately 8% higher than without the DFDD integrated into the IES.© 2004 ASME