Annual Energy Efficiency Research Articles

Performance evaluation of external compound parabolic concentrator integrated with thermal storage tank for domestic solar refrigeration system.

The aim of this research was to develop a model for a solar refrigeration system (SRS) that utilizes an External Compound Parabolic Collector and a thermal energy storage system (TESS) for solar water heating in Chennai, India. The system parameters were optimized using TRNSYS software by varying factors such as collector area, mass flow rate of heat transfer fluid, and storage system volume and height. The resulting optimized system was found to meet 80% of hot water requirements for the application on an annual basis, with an annual collector energy efficiency of 58% and an annual TESS exergy efficiency of 64% for a discharge period of 6h per day. In addition, the thermal performance of 3.5kW SRS was studied by connecting it to an optimized solar water heating system (SWHS). The system was found to generate an average cooling energy of 12.26MJ/h annually, with a coefficient of performance of 0.59. By demonstrating the ability to efficiently generate both hot water and cooling energy, the results of this study indicate the potential for utilizing a SWHS in combination with STST and SRS. The optimization of system parameters and the use of exergy analysis provide valuable insights into the thermal behavior and performance of the system, which can inform future designs and improve the overall efficiency of similar systems.

Model studies of a curved suction pipe of pumping units

The purpose of the research was to study the hydraulic conditions of the flow supply in the new design of the curved suction pipe and to study the effect of these conditions on the pump operation. Improving the suction pipes provides a predetermined schedule of water supply, reducing unproductive losses and the cost of electricity for water lifting. Therefore, such studies are relevant. The article uses methods of physical modeling with the introduction of the obtained data on full-scale structures. The values of the dimensionless coefficients and the corresponding hydraulic gradients for various times are studied on a suction pipe model at Qmax and Qmin, over the entire range of operating parameters. The chosen method of improving the suction pipes provides an optimal water supply schedule, reducing unproductive losses and electricity costs for water lifting. During the reconstruction of pumping stations, projects for the modernization of water supply facilities were introduced with annual energy efficiency amounting to 1312 million sum for the pumping stations Kuyumazor, Kiziltepa-1 - 105 million sum.

Optimization of Condensate Transfer Pump in the Hydrocarbon Condensate Stabilization Unit of Natural Gas Liquefaction Plant – An Industrial Case

PT. Badak NGL is a commercial LNG producer located in Bontang, Indonesia. In recent years, the decline in LNG production of PT. Badak NGL results in a lower hydrocarbon condensate flow rate. To prevent power losses since its pump operated at below rating, a new operation scheme was proposed. This new scheme adjusts the way of pump is operated without any alteration in terms of pump characteristics. Three pumps operated intermittently for 8 hours per day at a maximum flow rate. The pump power consumption was calculated based on electric current data monitored relay panel. The results show that the pump energy saved by 60% with the absolute value of annual energy efficiency of 957.7 MWh. This study contributes to reducing the greenhouse effect by 412 tons eq CO and the annual production cost of around USD 130,000. The new scheme reliability examines by evaluating pump vibration, pump performance, hydrocarbon condensate quality, and condensate tank liquid level. The result shows that the pump’s vibration in the range of 0.21 in/sec with a steady winding temperature.

Dynamic Investigation and Optimization of a Solar-Based Unit for Power and Green Hydrogen Production: A Case Study of the Greek Island, Kythnos

The aim of the present work is the analysis of a solar-driven unit that is located on the non-interconnected island of Kythnos, Greece, that can produce electricity and green hydrogen. More specifically, solar energy is exploited by parabolic trough collectors, and the produced heat is stored in a thermal energy storage tank. Additionally, an organic Rankine unit is incorporated to generate electricity, which contributes to covering the island’s demand in a clean and renewable way. When the power cannot be absorbed by the local grid, it can be provided to a water electrolyzer; therefore, the excess electricity is stored in the form of hydrogen. The produced hydrogen amount is compressed, afterward stored in tanks, and then finally can be utilized as a fuel to meet other important needs, such as powering vehicles or ferries. The installation is simulated parametrically and optimized on dynamic conditions, in terms of energy, exergy, and finance. According to the results, considering a base electrical load of 75 kW, the annual energy and exergy efficiencies are found at 14.52% and 15.48%, respectively, while the payback period of the system is determined at 6.73 years and the net present value is equal to EUR 1,073,384.

Optimization and Performance Analysis of a Distributed Energy System Considering the Coordination of the Operational Strategy and the Fluctuation of Annual Hourly Load

The operation strategies of a distributed energy system (DES) are usually proposed according to the electrical load (FEL) and the thermal load (FTL), which take the cooling/heating load or electric load as unique constraint conditions that result in a too high or too low equipment load rate. This paper proposes a new hybrid operation strategy (HOS) that takes the full utilization of natural gas and the minimization of power consumption from the power grid as constraints and coordinates the cooling/electricity ratio and heating/electricity ratio of buildings and equipment. In the optimization phase of a DES, an optimization method based on the discretization of the load is proposed to investigate the influence of the uncertainty of the load on the DES, which helps to avoid repeated load simulations and provides stronger adjustability by quoting the normal distribution function to obtain multiple sets of load data with different fluctuations. Further, a multi-objective optimization model combining the genetic algorithm (GA) and mixed integer linear programming algorithm (MILP) was established to find the optimal configuration of equipment capacities by comprehensively considering the annual total cost, carbon emissions, and energy efficiency of the DES. Finally, an office building example was used to validate the feasibility of the above theories and methods. Compared with the FEL and FTL, the HOS reduced the energy waste of the DES by 19.7% and 15.5%, respectively. Compared with using a typical daily load, using an annual hourly load to optimize the DES-HOS produced a better comprehensive performance and lower adverse impacts derived from load fluctuations.

Optimized radiative parameters of building roof surfaces for energy efficiency: Case studies in China

To improve the thermal performance of the roof and building energy efficiency, this study investigates the optimized combination of the shortwave absorptivity and longwave emissivity for exterior roof surfaces. A simplified 3-day field measurement was first performed for model validation. Next, simulations were performed in 12 representative Chinese cities with both radiative parameters varying between 0.1 and 0.9 for the exterior surface of un-insulated and insulated roofs, focusing on the thermal load of the building. Experiment confirmed that WUFI Plus could reliably process the radiative boundary condition, with the daily surface temperature difference between the measurement and simulation being approximately 1 °C. Furthermore, simulation showed distinct requirements for the radiative parameters of the building roof surface, with regional and seasonal differences being significant. In general, for cold areas and seasons, a combination of high shortwave absorptivity and low longwave emissivity is preferred. Conversely, for hot conditions, a low shortwave absorptivity combined with a high longwave emissivity is optimal. For scenarios with balanced heating and cooling loads, optimization can only be achieved through case-by-case analysis. Finally, a map of optimized radiative parameter combinations for building roofs is provided, accounting for annual building energy efficiency.

Heat pump integration in a real poly-generative energy district: A techno-economic analysis

• A time-dependent thermo-economic approach for energy districts analysis was used. • TES already proved their effectiveness as optimal storage asset of energy districts. • Integration of HP in a poly-generative district is simulated. • Compared to actual configuration, energy performance increases. • Power-to-heat systems based on HP + TES are valuable for energy districts management. This paper presents a techno-economic feasibility analysis related to a heat pump installation in a poly-generative energy district to convert the overproduction of electricity into thermal power, easy to be stored in thermal storage tanks. The heat pump technology is already used for thermal/cooling energy production in different areas although application in energy districts in a power-to-heat modality to improve management of electrical/thermal energy demands is still limited. In this research, the installation of a heat pump in the poly-generative smart grid located at the University of Genoa Campus is presented. A time dependent one-year techno-economic analysis of the energy district is performed, throughout a model built with a software developed by the authors. The integration of the heat pump in the energy district is analysed, comparing the energetic, environmental and economic performance to the present configuration of the poly-generative energy district. The results show that the heat pump introduction grants several advantages, such as a reduction in gas consumption (24 ton/year, −15%) and an increase in the annual energy efficiency of cogenerative prime movers which can work for a higher number of hours (+23%) close to the design point.

Evaluation of energy-saving retrofit projects of existing rural residential envelope structures from the perspective of rural residents: the Chinese case.

The existing residential envelope in rural areas has low energy efficiency, with low energy utilization and serious energy waste for winter heating. However, in recent years, farmers have not undertaken energy efficiency retrofit projects for existing buildings in rural areas. This study proposed an evaluation model based on the logistic-AHP-TOPSIS method from the perspective of farmers. First, this study conducted a questionnaire survey of 208 rural households and used logistic models to determine which existing evaluation indicators significantly impact farmers' willingness to participate in energy efficiency retrofit projects. Second, the weights of the eight indicators were determined using AHP. Finally, this study evaluated the retrofit program of a case in Gansu Province using the TOPSIS method. In the analysis, total investment, annual winter heating costs and energy efficiency improvements were found to be the most important factors for farmers. The highest score for existing buildings was 0.3747 because there is no additional investment required; the option of partial retrofitting according to the actual needs of households scored 0.3350 because it balances economic performance with energy efficiency performance; The entire retrofit program has the highest investment and a long payback period, and its score is the lowest with 0.2904. Furthermore, the study recommended that a self-build and self-repair organization led by village collectives be developed to unify the retrofit design and construction and lower renovation costs. Bulk purchases could enable farmers to increase their power in negotiations.

Seasonal and annual energy efficiency of mainstream anaerobic membrane bioreactor (AnMBR) in temperate climates: Assessment in onsite pilot plant and estimation in scaled-up plant

The seasonal and annual energy efficiency of mainstream anaerobic membrane bioreactor (AnMBR) was first assessed in an onsite pilot plant (15 m3/d) and then estimated in a scaled-up plant (10,000 m3/d) in temperate climates (15–25 °C). It was found that the annual net electricity demand was 0.100 and 0.090 kWh/m3, and the annual net energy (electricity + heat) demand was −0.158 and −0.309 kWh/m3 under the dissolved methane recovery condition and the non-recovery condition, respectively, demonstrated that the application of mainstream AnMBR in temperate climates is electricity saving and energy positive. The energy efficiency of the AnMBR decreased with temperature drop due to the reduction of methane production, and the increase in biogas sparging to mitigate membrane fouling. Since approximately 26.7%-39.7% of input COD remained in sludge, attention should be paid to recovering this potential energy to improve the overall energy performance of the mainstream AnMBR plants in future.

Optimization on annual energy efficiency of heat pumps based on maximum solving of definition functions with multi constraints

Air conditioners is expected to be the second largest source of growth in global electricity demand. To promote electrification, heat pump air conditioner is encouraged to replace natural gas furnace. Improving heat pump’s energy efficiency can help reduce global energy consumption and greenhouse gas emissions. Annual performance factor is one of the key metrics to evaluate heat pump systems’ energy performance. Seeking annual performance factor maximum belongs to an optimization problem with multi constraints. However, due to the nested variables, continuous non-differentiable and piecewise form of annual performance factor definition functions, the optimized objective function is usually based on the simplification with large errors. Meanwhile, the multi constraint boundaries are either ignored or considered but through complicated and high-cost tests. In this research, the variable iteration logic of the annual performance factor definition functions is sorted firstly. Then the above exact mathematical definitions are programmed as the objective functions without any simplification. Enumeration method is used to establish the annual performance factor maximum solution algorithm. More importantly, taking a 3.5 kW cooling capacity mini-split heat pump prototype as an example, energy performance tests are conducted to fit the multi constraints, namely using less global parameters to cover more local parameters, which greatly simplifies the test procedures and cost. Then, using our proposed method, the maximum annual performance factor is predicted and validated by experiments. The prediction accuracy is 99.6%, with a 38.4% improvement compared to the best reported value from literature. Based on the recommended variable values, the prototype’s annual performance factor achieves the maximum with a 7.71% improvement from the baseline. The proposed method can save time and experimental resources in the product development stage compared with the traditional trial and error method.

Power usage effectiveness analysis of a liquid-pump-driven hybrid cooling system for data centers in subclimate zones

Free cooling technology can effectively reduce the cooling energy consumption of data centers, but its energy efficiency is greatly affected by regional and meteorological conditions. This study investigates the liquid-pump-driven free cooling/mechanical refrigeration hybrid cooling system of a data center with 40 racks through a demonstration case and tests. The annual hourly energy efficiency in 20 subclimate zones and 37 typical cities in China of the hybrid cooling system designed with 5 and 10 °C temperature differences was analyzed with different meteorological parameters. The annual power usage effectiveness (PUE) values corresponding to the energy efficiency ratio (EER) in different regions were presented. Results showed that the annual average EER of the liquid-pump-driven free cooling mode was 7.07–14.18, and the annual average EER of the hybrid system was 4.22–14.10 when the temperature difference was 5 °C. The annual average EER range and the design temperature difference of the hybrid system for different climate regions were recommended. The PUE values of other regions, except for Guangzhou and Haikou, were lower than 1.40. The results provide references for the EER and PUE of the hybrid system in different climate regions.

Energy efficiency and environmental assessment of an integrated ground source heat pump and anaerobic digestion system

A hybrid system that integrates a ground source heat pump and an anaerobic digester (GPAD) was proposed and designed for rural buildings in the Hot Summer and Cold Winter (HSCW) region in China. The GPAD system can alleviate soil thermal imbalance and enable the anaerobic digester to produce biogas more efficiently during winter and transition seasons. To quantitatively assess such a system's energy and environmental performance, a two-story house located in Chongqing municipality was selected as a case study. Two conventional scenarios, including Ground Source Heat Pump (GSHP) and Air Source Heat Pump (ASHP) systems, were compared with the GPAD system. Results suggested that the annual energy efficiencies of the three scenarios were 3.66 (GSHP), 2.54 (ASHP), and 4.17 (GPAD), respectively; the GPAD system exhibited the highest efficiency, which was 14.0% higher than the GSHP system and 64.1% higher than the ASHP system. Meanwhile, the Thermal Imbalance Ratio (TIR) of the GPAD system was 32.2%, which was much lower than the GSHP system (53.5%). When considering the emissions avoided (avoiding treatment of manure and rice straw, consumption of chemical fertilizer and liquefied petroleum gas), Global Warming Potential (GWP) indicator was reduced from 0.34 kg CO2-eq (GSHP system) to 0.05 kg CO2-eq for unit heating and cooling. The total environmental impact index of the GPAD system was 99.2% lower than that of the GSHP system and 99.4% lower than that of the ASHP system. Besides, improving the Coefficient of Performance (COP) of the heat pump unit and integration with a photovoltaic system that generates electricity to power the operation of the GPAD system could significantly improve environmental performance.

Dynamic multi-objective optimization applied to a solar-geothermal multi-generation system for hydrogen production, desalination, and energy storage

The design of optimal energy systems is vital to achieving global environmental and economic targets. In the design of solar-geothermal multi-generation systems, most previous investigations have relied on the static multi-objective optimization approach (SMOA), which may leave considerable room for improvement under certain conditions. In this numerical study, the optimal condition at which to operate a solar-geothermal multi-generation system – which can simultaneously produce hydrogen, fresh water, electricity, and heat, along with storing energy − is determined via a dynamic multi-objective optimization approach (DMOA). Optimization is performed using a combination of NSGA-II and TOPSIS, and the results are benchmarked against those of SMOA. The decision variables include the solar area, geothermal water extraction mass flow, and hydrogen storage pressure. The objective functions include the production of electricity, heat, hydrogen, and fresh water, along with the exergy and energy efficiencies and the payback period. It is found that when compared with SMOA, DMOA can significantly improve all the objective functions. The annual production of electricity, heat, hydrogen, and fresh water increases by 14.4, 16.1, 13.5, and 14.3%, respectively, while the average annual exergy and energy efficiencies increase by 5.2 and 3.0%, respectively. The use of DMOA also reduces the payback period from 5.56 to 4.43 years, with a 4.4% reduction in hydrogen storage pressure. This shows that compared with a static approach such as SMOA, DMOA can improve the exergy and energy efficiencies, economic viability, and safety of a solar-geothermal multi-generation system.

A Novel Polygeneration System Based on a Solar-Assisted Desiccant Cooling System for Residential Buildings: An Energy and Environmental Analysis

This work aims to design and dynamically simulate a polygeneration system that integrates a solar-assisted desiccant cooling system for residential applications as an alternative to vapor compression systems. The overall plant layout supplies electricity, space heating and cooling, domestic hot water, and freshwater for a single-family townhouse located in the city of Almería in Spain. The leading technologies used in the system are photovoltaic/thermal collectors, reverse osmosis, and desiccant air conditioning. The system model was developed and accurately simulated in the TRNSYS environment for a 1-year simulation with a 5-min time step. Design optimization was carried out to investigate the system’s best configuration. The optimal structure showed a satisfactory total annual energy efficiency in solar collectors of about 0.35 and about 0.47 for desiccant air conditioning. Coverage of electricity, space heating and cooling, domestic hot water, and freshwater was 104.1%, 87.01%, 97.98%, 96.05 %, and 100 %, respectively. Furthermore, significant ratios for primary energy saving, 98.62%, and CO2 saving, 97.17%, were achieved. The users’ thermal comfort level was satisfactory over the entire year. Finally, a comparison with an alternative coastal site was performed to extend the polygeneration system’s applicability.

Design and performance assessment of a solar-to-hydrogen system thermally assisted by recovered heat from a molten carbonate fuel cell

Solar-to-hydrogen plants are predominantly based on steam electrolysis. Steam electrolysis requires water, electricity and heat. The excess electric energy is generally converted into hydrogen via an electrolyser. The use of waste heat in hydrogen generation process promises energy efficiency improvement and production fluctuation reductions. This work investigates the techno-economic performance of the proposed system which recovers the waste heat from molten carbonate fuel cell and uses solar energy to produce steam. Comparison of thermally assisted solar system with corresponding solar system is done. The fuel cell provides 80% of the required thermal energy. The solar PV array provides the required electricity. The thermally assisted solar-to-hydrogen system annual energy efficiency (38.5%) is higher than that of solar- to- hydrogen system. The investment cost of the proposed system is 2.4% higher than that using only solar parabolic trough collector for the same required amount of heat. The advantage is that the fuel cell simultaneously produces electricity and heat. The recovery of waste heat allows getting an annual overall efficiency of 63.2% for the molten carbonate fuel cell. It yields 2152 MWh of electricity per year. The 1 MW electrolysers annually generates 74 tonnes of hydrogen.

Investigation and optimization of a CO2-based polygeneration unit for supermarkets

The objective of the present analysis is to investigate and optimize a solar-driven polygeneration system with CO2 as the working fluid. The studied idea is ideal for installation in supermarkets in order to produce refrigeration, electricity and heating. More specifically, the primary energy source of this system is solar irradiation and the unit produces, at the same time, cooling to cover the refrigeration demands of the store products, useful heat to cover the heating demands and electricity to cover the consumption of the refrigeration units and for sale to the national grid. Therefore, the proposed system includes a parabolic trough collector field, a storage tank, an upgraded CO2 cycle and an organic Rankine cycle. The CO2 cycle is based on an existing CO2 refrigeration cycle and includes additional Brayton cycle devices for power production. The unit is investigated and optimized in steady-state conditions and then the most suitable design is studied in dynamic conditions for all the year. According to the final results, the annual energy efficiency of the system is 67.8% and the annual exergy efficiency is 10.1%. Furthermore, the net present value is calculated to be equal to 543 k€ and a payback period of 7.55 years is found. So, apart from the food refrigeration, the other needs of the supermarket, such as electricity and heating, are met by a system, which is highly efficient and economically viable.

Pavement integrated photovoltaic thermal (PIPVT) system: A temporal and spatial analysis of energy and exergy performance

Pavement integrated photovoltaic thermal (PIPVT) technology can utilize vast roads and pavements for electrical and thermal energy generation simultaneously. In this paper, a two-dimensional transient numerical model is developed for demonstrating the performance of a PIPVT system with two PIPVT modules and a 50 L water tank. With the mathematical model, the system performance is examined using simulations under three typical weather conditions, namely sunny, semi-cloudy, and cloudy. Besides, a long-term simulation is conducted in five climatic regions represented by five cities in China, including Lhasa, Harbin, Haikou, Shanghai, and Yinchuan, to study the seasonal and annual performance of PIPVT systems in different areas. From the viewpoint of both the first and second laws of thermodynamics, both energy and exergy analyses are performed for all the cases in different weathers and climatic regions. The overall energy efficiencies of sunny, semi-cloudy, and cloudy days are 33.10%, 34.74%, and 18.60%, respectively, revealing that the PIPVT system performs well on sunny days and semi-cloudy days, while the overall performance is unsatisfactory on cloudy days with extremely low solar radiation that rarely appears. Particularly, among the five climatic regions, the highland climatic region is ideal for PIPVT application with both the highest annual overall energy and exergy efficiencies of 37.89% and 14.17%, respectively. The icing phenomenon of the water in tubes arises in winter and late autumn in both the temperate monsoon climatic region and the temperate continental climatic region. The overall performance of PIPVT systems is best in western and southern China.

Experimental investigation and application analysis on an integrated system of free cooling and heat recovery for data centers

Rapid growth of data centers has caused a huge increase in energy consumption. Utilizing natural cold resources and harvesting waste heat are two promising ways to reduce the total power consumption of data centers. This paper proposes an integrated system of free cooling and heat recovery. The system has four working modes: mechanical cooling, mechanical heat recovery, pump free cooling and pump heat recovery. Experimental results show that pump free cooling mode can achieve an energy efficiency ratio of 7.99 when outdoor temperature is 2 °C, and different modes can be switched smoothly. The applicability of this system has been evaluated in five different cities in China. The results show that annual average energy efficiency ratio can reach 6.18 and free cooling time can take up as high as 72.9% during operation in Xining. The system can harvest the most heat in Harbin, 7930.96 kW∙ h per year, while the least in Guangdong, 645.31 kW∙ h per year. This means that this new system has relatively great application prospects in northern cold cities of China, while is moderately limited in southern cities.

Synthesis of heat exchanger network with complex phase transition based on pinch technology and carbon tax

The aim of this paper is to integrate carbon emissions into the synthesis of heat exchanger network for an industrial process with a four-stage pyrolysis gas compression cycle and depropanizers. Problem table is used to determine the pinch point for the design of heat exchanger network. Due to the complex phase transition of the mixture in the system, the phase transition is considered as a pseudo-stream with constant heat capacity flow rate according to its thermal load and material characteristics within several individual temperature intervals. The contribution of carbon tax is also introduced to the total annual cost to determine the suitable minimum approach temperature(△Tmin). Compared with traditional pinch point method, △Tmin is determined as 8°C instead of 9°C due to the consideration of carbon emission. The pinch point is obtained at 80.15°C for hot streams and 72.15°C for cold streams, under which the minimum hot and cold utilities required by the heat exchanger network are 28046.89 kW and 69324.05kW respectively. The total annual cost is saved by 4,446,559 USD.a−1 and the carbon emission is reduced by 96689.31t/a. Results indicate that the consideration of carbon emissions based on carbon tax is helpful to obtain a better heat exchanger network in terms of total annual cost and energy efficiency on the way to carbon neutrality.

Comparative study of the electrical performance of three types of photovoltaic technologies in Rabat and Errachidia station

The production of electricity by the photovoltaic systems is an idea that has been applied for many years. In this paper, the annual AC power, total AC power of three kinds of photovoltaic systems (Monocrystalline, polycrystalline, amorphous silicon) were calculated and discussed in Rabat and Errachidia stations, located approximately 316.61 km apart, over the year 2018. The photovoltaic (PV) systems were installed in order to determine which technology produces the most clean electricity. It is based on a comparison of the monthly production of the three technologies in the two cities, and the justification of the behaviour of the three technologies using two similar weather stations installed near the solar panels. In such a way, the determination of the climatic conditions will justify the behaviour of the three technologies. The comparative analysis included annual energy efficiency, performance ratio, annual energy density and system efficiency. Thus, with these performances, we can observe that photovoltaic installations with polycrystalline are appropriate in Errachidia, whereas in Rabat they are monocrystalline. Also, the photovoltaic system of Errachidia stations is more efficient as compared with Rabat stations.