• Energy Research

AbstractThis study presents an optimized modelling approach for ORC based on radial turbo-expander, where the constant expander efficiency is replaced by dynamic efficiency and is unique for each set of cycle operating conditions and working fluid properties. The model was used to identify the key variables that have significant effects on the turbine overall size. These parameters are then included in the optimization process using genetic algorithm to minimize the turbine overall size for six organic fluids. Results showed that, dynamic efficiency approach predicted considerable differences in the turbine efficiencies of various working fluids at different operating conditions with the maximum difference of 7.3% predicted between the turbine efficiencies of n-pentane and R245fa. Also, the optimization results predicted that minimum turbine overall size was achieved by R236fa with the value of 0.0576m. Such results highlight the potential of the optimized modeling technique to further improve the performance estimation of ORC and minimize the size.

Abstract The availability of potable water and cooling are becoming increasingly important to ensure good sustainability and quality of life. In this work, a new multi-cycle adsorption desalination and cooling system using AQSOA-Z02 has been developed for high water production and cooling rates using renewable and waste heat sources. It consists of two 2-adsorber bed cycles linked with integrated evaporator/condenser, one cycle uses the integrated evaporator/condenser as evaporator (upper) and the second one uses it as a condenser (lower). In this system low condensing temperatures can be achieved using the cooling effect from the evaporator of the lower cycle and the integrated evaporator/condenser thus enhancing the system performance. Also, the adsorber beds of the upper and lower cycles are heated in series during the desorption process using the same heat source. This system can operate in three modes depending on the desalinated water and cooling capacity requirements. Results showed that the specific daily water production ranges from 6.64 to 15.4 m3/tonne adsorbent/day while the cooling capacity reaches up 46.6 Rton/tonne adsorbent at evaporator temperature of 10 °C. The new cycle offers potential of simultaneously producing large amounts of desalinated water and cooling capacity (at 10 °C) compared to other cycles.

Abstract With ever increasing demand on energy, disturbed power generation utilizing efficient technologies such as compressed air energy storage (CAES) and organic Rankine cycle (ORC) are receiving growing attention. Expander for such systems is a key component and its performance has substantial effects on overall system efficiency. This study addresses such component by proposing an effective and comprehensive methodology for developing a small-scale radial inflow turbine (RIT). The methodology consists of 1-D modelling, 3-D aerodynamic investigation and structural analysis, manufacturing with pioneering technique and experimental testing for validation. The proposed 1-D modelling was very effective in determining the primary geometry and performance of turbine based on parametric studies of turbine input design variables. However with CFD analysis, it was shown that more efficient turbine geometry can be achieved that not only provides more realistic turbine performance by capturing the 3-D fluid flow behaviour but also improves turbine efficiency with the aid of parametric studies of turbine geometry parameters. Turbine efficiency was improved from 81.3% obtained from 1-D modelling to 84.5% obtained by CFD. Accuracy of the CFD model was assessed by conducting experiments on the RIT manufactured with stereolithography technique. The CFD model can predict turbine efficiency and power with accuracy of ±16% and ±13% respectively for a wide range of tested operating conditions. Such results highlights the effectiveness of the proposed methodology and the CFD model can be used as benchmarking model for analyses of small-scale RITs. Besides, it was shown that for such applications, the novel manufacturing technique and employed material are very effective for producing prototypes that assist design decisions and validation of CFD model with reasonable accuracy at reasonable cost and in timely manner.

Abstract Current air conditioning (AC) systems consume a significant amount of energy, particularly during peak times where most electricity suppliers face difficulties to meet the users’ demands, and the global demands for AC systems have increased rapidly over the last few decades leading to significant power consumption and carbon dioxide emissions. This paper presents a new technique that uses liquid nitrogen (LN2) produced from renewable energy sources, or surplus electricity at off peak times, to provide cooling and power for domestic houses. Thermodynamic analyses of various cryogenic cycles have been carried out to achieve the most effective configuration that produces the maximum power output with minimum LN2 flow rate, to meet the required cooling of a 170 m2 dwelling in Libya. A comparison with a conventional AC system was also made. Results showed that at the current LN2 prices, using LN2 to provide cooling and power demands of residential buildings is feasible and saves up to 36% compared to conventional air conditioning systems with an overall thermal efficiency of 74%. However, as the LN2 price decreases to around 1.3 pence per kg, the proposed technology will have significant advantages compared to conventional AC systems with savings of up to 81%.

Abstract Adsorption desalination is a promising technology that has recently been investigated. Most of the reported adsorption desalination systems use silica gel as the adsorbent and despite the high stability, it suffers from limited water uptake capabilities leading to a low system performance. Metal-organic frameworks (MOFs) are porous materials with high surface area, pore size, tunable pore geometry and hence providing high adsorption capacity. Currently, limited MOF materials with high water adsorption capabilities and hydrothermal stability are commercially available. CPO-27(Ni) and aluminium fumarate are two commercially available MOFs that have a maximum water uptake of 0.47 gH2O·gads− 1 and 0.53 gH2O·gad− 1, respectively. Another MOF, MIL-101(Cr), exhibits superior water adsorption uptake of 1.47 gH2O·gad− 1 but currently can only be produced in lab-scale. The thermodynamic cycle performance of a two beds adsorption system was evaluated using Simulink software to assess the suitability of those MOFs for adsorption desalination and their performance under different operating conditions. The CPO-27(Ni) was found to produce around 4.3 m3·(ton·day)− 1 at an evaporation temperature of 5 °C while aluminium fumarate produced around 6 m3·(ton·day)− 1 at an evaporation temperature of 20 °C. As for MIL-101(Cr), the water production rate at 20 °C was 11 m3·(ton·day)− 1 highlighting the potential of this material compared to other adsorbents.

Abstract A cascaded adsorption cooling system with an integrated evaporator/condenser can produce low temperature cooling, using waste heat sources. The choice of the working pair in such a system affects the system’s performance when driven by low temperature waste heat sources, which can be as low as 70 °C. This paper investigates the performance of various adsorbent/refrigerant working pairs in a cascaded adsorption system with an integrated evaporator/condenser using Simulink/MATLAB software. The cascaded system consists of two pairs of adsorber beds, a condenser, an evaporator and an integrated condenser/evaporator heat exchanger, forming upper and bottoming cycles. Five combinations of working pairs were investigated: ATO/ethanol + Maxsorb/R507A; Maxsorb/R134a + Maxsorb/propane; ATO/Ethanol + Maxsorb/propane; ATO/ethanol + AC-35/methanol; and Maxsorb/R134a + Maxsorb/R507A. The latter combination was used for validation and as a reference combination for assessing the performance of the investigated working pairs in terms of COP and cooling capacity. The results showed that the Maxsorb/R134a + Maxsorb/propane combination gives a higher COP compared to the reference combination, with up to 30.0% and 30.1% for the COP and cooling capacity, respectively; while ATO/ethanol + AC-35/methanol produces a similar performance to the reference case but uses natural refrigerants with low global warming potential and low cost adsorbent materials.

Abstract Globally there is abundance of low grade heat sources (around 150 °C) from renewables like solar energy or from industrial waste heat. The exploitation of such low grade heat sources will reduce fossil fuel consumption and CO 2 emissions. Adsorption technology offers the potential of using such low grade heat to generate cooling and power. In this work, the effect of using advanced adsorbent materials like AQSOA-Z02 (SAPO-34) zeolite and MIL101Cr Metal Organic Framework (MOF) at various operating conditions on power and cooling performance compared to that of commonly used silica-gel was investigated using water as refrigerant. A mathematical model for a two bed adsorption cooling cycle has been developed with the cycle modified to produce power by incorporating an expander between the desorber and the condenser. Results show that it is possible to produce power and cooling at the same time without affecting the cooling output. Results also show that for all adsorbents used as the heat source temperature increases, the cooling effect and power generated increase. As for increasing the cold bed temperature, this will decrease the cooling effect and power output except for SAPO-34 which shows slightly increasing trend of cooling and power output. As the condenser cooling temperature increases, the cooling effect and power output will decrease while for the chilled water temperature, the cooling load and power generated increased as the temperature increased. The maximum values of average specific power generation (SP), specific cooling power (SCP) and cycle efficiency are 73 W/kg ads , 681 W/kg ads (using SAPO-34) and 67% (using Silica-gel) respectively. However, MIL101Cr can generate SP and SCP of 95 W/kg ads and 1367 W/kg ads respectively, but this case cannot consider to be practical operating conditions, because of using relatively low cooling source temperature, but this material still offers potential of generating power.

Abstract Conventional solar compound parabolic concentrators are normally fitted with one tubular receiver positioned along the axis of the two parabolas. This work investigates the potential of using either two tubular receivers in one compound parabolic concentrator or an elliptical single receiver. Using advanced ray tracing technique, the optical efficiency of a compound parabolic concentrator with two tubular receivers aligned horizontally and vertically was predicted. Results showed that the horizontal configuration outperforms both the single and the vertical configurations by up to 15%. Also, a horizontally aligned elliptical single tube showed an increase in the average daily optical efficiency by 17% compared to the single tube configuration. The thermal performance of the single and double horizontally aligned tubular receivers was determined using a thermosyphon heat pipe experimentally tested utilising the heat flux obtained from the optical simulation at different acceptance angles. Results show that double tube configuration thermally outperforms the single one in terms of heat transferred to the cooling water by 21%, 19.8% and 18.3% for acceptance angles of 30°, 40° and 60° respectively. This work highlights the potential of using either two tubular receivers or single elliptical one aligned horizontally in one concentrator to improve the optical and thermal efficiencies of compound parabolic collectors.

Abstract This work presents the development and validation of computational fluid dynamics (CFD) model of 500 W gamma-type Stirling engine prototype to highlight the effects posed by phase angle and dead volume variations on engine performance. The model is based on a realistic Local Thermal Non-Equilibrium (LTNE) approach for porous domains in the engine (cooler and regenerator). The simulation results showed an acceptable degree of accuracy of 9% and 5%, respectively when comparing with experimental results in predicting the indicated and cooling powers at different heating temperatures. It is found that the maximum indicated power is achieved at a phase angle of 105° rather than at the common phase angle of 90°. The dead volume (connecting pipe) is observed to pose negative effects on engine indicated power and therefore, an optimum value of pipe diameter exists.