Low Pressure Receiver Research Articles

Experiments and dynamic modelling of dry expansion and flooded evaporators in a CO2 solar assisted heat pump

• A direct-type SAHP working with CO 2 and three PV-T collectors is presented. • Flooded evaporation, obtained with natural circulation, avoid mal-distribution issues. • When superheating is present, the performance drops due to lower collector efficiency. • A dynamic numerical model of the heat pump has been realized in Matlab environment. • The model is used to study solutions to control and avoid superheating. In the present study, a dynamic model of evaporators in a CO 2 solar assisted heat pump is presented and experimentally validated. The evaporators are PV-T (photovoltaic-thermal) solar collectors and they can work both in direct expansion mode and flooded mode. Thanks to the PV-T evaporators we convert the solar radiation into electricity in the PV cells, ensure the evaporation of the CO 2 and cool the solar cells to improve the PV conversion efficiency. The evaporators of the heat pump can work under two operative modes, dry expansion and flooded evaporation. When working in dry-expansion mode, the refrigerant mass flow rate after the throttling valve is directly sent to the solar collectors. Alternatively, when operating in flooded mode, the collectors are fed with saturated liquid refrigerant coming from a low-pressure receiver. In real applications, we can have multiple collectors that are fed in parallel and thus the advantage and the main reason for studying the flooded solar evaporators is that, with this schematic, we can feed them with saturated liquid avoiding possible distribution problems that may occur when distributing the liquid-vapor mixture (direct expansion mode). In this article, the operating mechanism of the heat pump is presented and an experimental comparison between the dry expansion and flooded evaporation is discussed. Since the system operation strongly depends on the solar irradiance and its variation, a comprehensive dynamic numerical model of the system is developed and validated. The results of the model show the regulation mechanism of the evaporators and the whole system and allow to study the solution to possible issues that are encountered in this application.

Dissimilar weld failure: A forensic analysis to determine primary failure mechanisms

Solar receivers are an integral part of a concentrated solar power plant and commonly utilise tubular structures to absorb solar energy and transfer the heat into a heat transfer fluid. These systems often contain dissimilar materials joined through welds which are exposed to cyclic temperatures, which can be a locus of failure. A systematic forensic analysis was carried out on a low-pressure CO2 receiver that had developed extensive cracking. Microstructural characterisation using micro-computed tomography was performed to understand the failure mechanism in an area adjacent to a welded section of the two dissimilar alloys Haynes 230 and 253 MA. An electrolytical oxalic acid etch showed grain boundary damage from oxidation. Grain boundary damage through oxidation was confirmed with SEM and energy dispersive X-ray spectroscopy (EDX) analysis as the likely metallurgical degradation mechanism which, combined with thermally induced stress cycles led to the failure of the weaker stainless-steel tube adjacent to the weld.

Modelling the performance of a new cooling unit for refrigerated transport using carbon dioxide as the refrigerant

• Development of a dynamic numerical model of a CO 2 cooling unit. • 3 layouts, including a back-pressure valve, ejector and auxiliary evaporator. • The ejector can give a COP increase of +15.9% at T amb = 42 °C, T i = −5 °C. • The auxiliary evaporator can extend the operating range of the ejector to lower T amb . • The auxiliary evaporator gives a max COP increase of 21.0% for T amb = 25 °C, T i = 5 °C. This paper provides a theoretical assessment of the thermal performance of a new CO 2 vapour-compression system for refrigerated transport applications. Three different configurations are investigated: the standard back-pressure with low pressure receiver lay-out and two arrangements integrating a two-phase ejector. In particular, the use of an auxiliary evaporator in the outlet line of the ejector is considered, to extend the ejector operating range. A numerical model of the system is developed and its theoretical performance is discussed for different values of internal space temperature and external ambient temperature. Simulations’ results show that the ejector cycle configuration is convenient when the system is operating in a hot climate with a maximum COP increase (compared to the traditional configuration) equal to 15.9%, at 42 °C ambient temperature and -5 °C internal space temperature. The use of an auxiliary evaporator can extend the operating range of the ejector to lower values of ambient temperature, with a maximum COP improvement (over the traditional configuration) equal to 21.0% at 25 °C ambient temperature and 5 °C internal cargo space.

The SUNDISC cycle: A direct storage-charging dual-pressure air receiver cycle

A novel concentrating solar power cycle, the SUNDISC cycle, is proposed to address identified shortcomings of asynchronous combined cycles with a solarized gas turbine. The bottleneck in these cycles is the gas turbine because the mass flow through the pressurized air receiver system and, therefore, the amount of thermal energy that can be used in the bottoming cycle, is limited to the maximum throughput of the turbine. The enhancement in the SUNDISC cycle is an additional low-pressure air receiver system that is uncoupled from the gas turbine and directly feeds the storage system or steam generator.Different variations of the cycle with separated and hybrid receiver systems are presented and selected operating modes are shown. The asynchronous power cycles and separated receiver systems allow for project-specific plant layout and operation.Annual simulations with an hourly thermodynamic model returned the lowest levelized cost for a plant generating electricity during 87% of the year practically without co-firing. During almost all of this time, one of the two cycles operates at full load. The large storage capacity needed for this baseload plant is viable because of the low anticipated cost of the rock bed thermal energy storage technology. The calculated levelized cost of this plant is 25% lower than for a plant without a low-pressure receiver and is predicted to be on a par with a next-generation molten salt plant’s cost once medium-scale roll-out has commenced.

A Cost and Performance Evaluation of SUNDISC: A Dual-pressure Air Receiver Cycle

Abstract A dual-pressure air receiver cycle has previously been proposed to overcome limitations of combined cycle CSP plants. The economic and energetic performances of this so-called SUNDISC cycle are investigated in the present work. To do so, an annual hourly thermodynamic model is used with varying configurations of solar field size, steam turbine rating, storage capacity, low-pressure receiver system and hybridization modes. The conducted simulations show the lowest levelized cost of electricity for small solar components and considerable fuel co-firing rates at 0.08 USD/kWhe. However, the results were produced with exceptionally low fuel cost and, furthermore, solar power plants are expected to be limited to low fuel usage. When the co-firing rate is capped at 20% and 5%, the minimum achievable costs increase to 0.10 USD/kWhe and 0.12 USD/kWhe, respectively. These latter results are generated with high steam turbine ratings as well as large low-pressure receiver systems, solar fields and thermal energy storage systems. The resulting annual time of no power generation, as a measurement of the plant's baseload capacity, was less than 800 h. If longer plant operation is aspired either cost or fuel co-firing rate increase. The rock bed thermal energy storage system is found to add only minor costs to the plant. Large storage capacities are, therefore, viable and the technology plays a vital role in enabling baseload electricity generation from solar energy. Refinements are recommended for the component cost model and plant operation control. The latter should be made more flexible to find optimized hybridization modes.

Dual-pressure Air Receiver Cycle for Direct Storage Charging

Combined cycle solar thermal power plants are expected to bring an efficiency increase as compared to state-of-the-art cycles. A passive sensible heat storage system in the low pressure part is expected to enable dispatchability of such a plant. However, the amount of energy available for the bottoming cycle is limited to the exhaust energy of the gas turbine. This is not sufficient if high solar shares and the generation of large amounts of electricity throughout the day and night are aspired.SUNDISC, a cycle with a dual-pressure receiver system, is proposed to address this limitation. Energy dumping through defocusing can be replaced by refocusing onto the low-pressure receiver system, which directly charges the storage system and, therefore, allows more energy to be harnessed during daytime. This potentially enables the bottoming cycle to be of comparable rating to the top cycle while still running throughout the night.A comparison of a non-optimized SUNDISC cycle and a layout without the low-pressure receiver system showed a 10% larger annual power generation of the former. Further improvements in solar share, power generation and dispatchability are expected for systems that are optimized for the respective requirements. Additional advantages in the areas of heliostat field optics and operational consistency have yet to be quantified.

Experimental analysis of a new method for overfeeding multiple evaporators in refrigeration systems

An innovative method for feeding flooded evaporators, arranged in parallel in R744 plants, is presented. In order to promote evaporators overfeeding, an ejector circulates liquid from the low pressure receiver back to the intermediate pressure receiver. Since there is not a need for superheat at the evaporator exit, each evaporator expansion valve can be controlled by a signal representative, either of the evaporator load, or the evaporator performance, such as the air temperature at its inlet or outlet. When compared to dry-expansion evaporator systems, the proposed solution offers energy saving and smooth operating conditions, typical of flooded evaporators, while still maintaining a simple plant lay-out. The concept was experimentally validated: the evaporators were maintained in flooded conditions, while the ejector was able to promote liquid recirculation. Direct comparison with dry-expansion operations was performed. The suggested method is mainly intended for commercial refrigeration plants, where many evaporators are arranged in parallel.

An experimental and theoretical study of heat and mass transfer during the venting of gas from pressure vessels

Non condensing gas flow and heat transfer during venting of vessels are studied using experiments and analysis. A high pressure helium supply vessel is connected to a low pressure receiver via orifice and tubing. A single control volume analysis and a multi-dimensional analysis are used to predict pressure and mass-averaged temperature in the supply. Experiments utilizing transient PVT methods are conducted to obtain transient pressure and mass-averaged temperature data for validating the analysis. Measured transient pressures and mass-averaged temperatures in the supply are reproduced by analysis. Heat transfer is due to natural convection except for the early part of transfer.

Theoretical and experimental analysis of a CO 2 heat pump for domestic hot water

This paper describes the development of a CO 2 air/water heat pump for the production of tap hot water in a residential building. The basic design consists of a single-stage piston compressor, a coaxial type gas cooler, an electronic expansion valve, a finned tube evaporator and a low pressure receiver. The heat pump is combined with a storage tank designed to maintain internal water stratification. The gas cooler pressure optimisation in the case of fixed water delivery temperature was theoretically analysed. A new control method for the upper cycle pressure was developed to maximise the COP of the heat pump, while the water mass flow was adjusted to maintain the set water temperature at the gas cooler exit. Before commissioning, the heat pump was factory tested to verify its energy performance and to validate the high pressure control logic.

Development of a novel multi-capillary, multi-temperature commercial refrigerator cabinet with common low-pressure receiver

A multi-temperature 4 drawer catering cabinet was designed to operate using a low-pressure receiver with capillary expansion to the separate evaporator in each drawer. Low-pressure receivers have been shown to be an effective way of allowing evaporators to operate in a fully flooded mode thus enabling more efficient use of the evaporator surface for heat transfer. If a low-pressure receiver is used in a refrigeration circuit the control of refrigerant flow into the evaporator is less critical as the expansion device is not responsible for preventing liquid returning to the compressor. Therefore, a capillary expansion device can be used effectively over a range of operating pressures. The system was shown to be effective at maintaining temperatures in the storage drawers during chilled, frozen and mixed storage temperature tests carried out to the EN441 test standard. The cabinet operated successfully at all conditions except when the heat load in each drawer was excessive (>400 W above base level heat load). In this case, refrigerant was found to back up in the condenser and the low-pressure receiver was empty of liquid refrigerant. A solution to this would be to allow controlled flow of refrigerant from the condenser to the low-pressure receiver at high condensing pressures.

Refrigeration systems with minimum charge of refrigerant

Concern for the environmental effects of HFC-refrigerants as well as the use of flammable refrigerants has resulted in a need of decreasing the refrigerant charge in refrigeration and heat pump systems. This paper discusses the possibility of such reductions, both at the systems- and the component level. It is shown that a move towards indirect systems, using secondary refrigerants, on both the cold and the hot side of the system may result in considerable reduction of charge. However, this reduction may come at the cost of slightly reduced system performance, which in itself is detrimental from an environmental point of view. At the component level, it may be shown that the main contents of refrigerant is usually contained in the heat exchangers. By selecting compact designs the charge may be reduced to extremely low levels. Specifically, mini-channel heat exchangers can be used for reaching low charge. With proper selection of heat exchangers, the system performance should not be influenced by the reduction of charge. For indirect systems, the amount of refrigerant solved in the compressor oil may be comparable to the amount in the (compact) heat exchangers. A possible solution to reduce this amount is to use compressors with less oil. With components selected for minimum charge, the system design may be different than what is usual. Instead of a high pressure receiver and a thermostatic expansion valve, a capillary tube may be used in combination with a minimal low pressure receiver, similar to the system design used in household refrigerators.

Metering of Steam-Water Two-Phase Flow by Sharp-Edged Orifices

This paper describes work conducted on the passage of steam-water mixtures through standard sharp-edged orifices for the prediction of flow conditions. When the stagnation enthalpy or alternatively the dryness fraction at the orifice is known, it is shown that the mass flow rate can be approximately determined. For the case where a steam-water mixture passes through a pipeline to the atmosphere or to a low-pressure receiver so that critical flow occurs at the outlet, both the mass flow rate and the stagnation enthalpy may be approximately evaluated using an orifice meter in series with a pressure tapping located at the point of critical flow. In this case, a trial-and-error method is necessary. The experimental results were within a range of dryness fractions from 1 per cent to 56 per cent, orifice pressures from 75 to 275 lb/in2 abs., differentials from 15 to 780 mmHg under water and flow rates from 54 700 to 508 000 lb/h. Orifices of 5.591 and 6.615 in diameter were installed in 7.9-in diameter pipes which were equipped with radius ( D and D/2) taps. The method employs a correlation between the homogeneous dryness fraction x, and a corrected dryness fraction x m necessary for evaluating the two-phase density used in the conventional single-phase meter equation.