Isothermal transient ionic current study of laminated electrochromic devices for smart window applications

The isothermal transient ionic current (ITIC) in three different kinds of laminated electrochromic devices has been determined. The devices consisted of one 300 nm thick layer of nickel oxide and one 300 nm thick layer of WO₃ deposited onto separate In₂O₃:Sn (ITO) covered glass substrates by DC magnetron sputtering at room temperature. They were then laminated with a polymeric ion conductor, acting as electrolyte, in symmetric and asymmetric configurations, i.e. WO₃/WO₃, NiO_xV_yH_z/NiO_xV_yH_z and WO₃/NiO_xH_y. The electrolyte was prepared by mixing polyethylene glycol of average molecular weight 400 (PEG 400) and lithium triflouromethanesulfonate (LiSO₃CF₃) for 12 h at 70°C and with a ratio oxygen/lithium (O/Li)=10. The samples were first polarized, i.e. the ions are transported to one of the electrodes, which in the asymmetric devices is the nickel oxide electrode. At the first applied potential step, the ions move through the electrolyte towards the opposite electrode. The potential is then switched and the ions move back to the first electrode. The ITIC curves are found to depend on the electrode material and in the asymmetric case also the direction of the ion current.     

Smart switchable glazing for solar energy and daylight control

The exciting field of chromogenic materials for smart windows and other large-area applications is discussed. A selection of switchable glazing devices that change color electrically are detailed. The types of devices covered are the electrochromic which change color electrically, covering electrochromic, dispersed liquid crystal and dispersed particle glazing that switch under an applied electric field. Device structures and switching characteristics are compared. The status of prototype and commercial devices from commercial and university labs through out the world are covered. A discussion of the future of this technology is made including areas of necessary development for the realization of large area glazing in excess of 1m².     

Evaluation of solar cooker thermal performance using different insulating materials

This paper summarizes very briefly the thermal performance of solar cookers with four different insulations readily available in rural areas. A comparison of each one of these is made with the performance of glass wool. This experiment is performed to minimize the cost of the cooker with a view to enhance its widespread application in the rural Indian environment.     

A numerical comparison of control strategies applied to an existing ice storage system

While ice storage systems are designed according to a defined strategy for warm day loads, it is interesting to consider other conventional control strategies for mid-season day loads. Three different charging–discharging control strategies are applied to an existing cooling plant and compared in terms of operating costs and energy consumption. A cooling plant model is built. A time stage equal to 15 min is considered to simulate numerically a whole charging–discharging process and compare the different control strategies. These simulations take into account existing technical constraints and set points. EES software is used. The operating costs of the cooling plant are evaluated by taking into account both the energy and the demand cost rate. It is shown that an ice storage system can allow savings of operating costs. However, they can increase energy consumption.     

Hydrogen fuel as an important element of the energy storage needs for future smart cities

A considerable amount of non-dispatchable photovoltaic and wind power have always been planned in smart cities, however, the problem of massive energy storage has not yet been solved which limits the use of green energy on larger scale. At present the only battery energy storage is available, and it is effective only for storing modest quantities of energy for short periods of time. The other storage technology options are not often commercially available items; rather, they are just good concepts that need to be tested for viability. Currently, the only alternative options for turning an urban development into one that exclusively uses green energy is to use that energy to generate hydrogen through electrolyzers, then use this fuel to generate the required electricity in order to stabilize the grid. Even more appealing is the idea of using wind and photovoltaic energy to transform smart communities into a centre for producing hydrogen in addition to a city that solely uses renewable energy. The most likely solution, absent an urgent debate inside the science establishment, will be to import electricity from the burning of hydrocarbons while continuing to pay carbon offsets, which is incompatible with the goal of using only renewables. The smart city has not officially accepted this issue, just like the science establishment.     

Experimental results of different control strategies in a solar air-conditioning system at part load

In this article, different control strategies used in a solar cooling installation are presented and compared, publishing real working data from three summer periods, with the system working at part load.The facility consists of two solar fields that feed a heating system during the winter and a 35 kW Li-Br absorption chiller during the summer period to cool part of an office building.The improvements obtained using control based on critical radiation and a variable flow mass have been compared on the solar plant with a conventional constant flow control. In the same way, the improvements obtained in the yields of a cooling production plant have been compared when programming three different controls: the first one with fixed flow masses, the second adapting the temperature on the condenser as a function of the generator temperature (constant flow mass), and the third adapting the condenser temperature and the flow mass on the generator as a function of the system loads.     

Optimization of control strategies for stand-alone renewable energy systems with hydrogen storage

This paper presents a novel strategy, optimized by genetic algorithms, to control stand-alone hybrid renewable electrical systems with hydrogen storage. The strategy optimizes the control of the hybrid system minimizing the total cost throughout its lifetime. The optimized hybrid system can be composed of renewable sources (wind, PV and hydro), batteries, fuel cell, AC generator and electrolyzer. If the renewable sources produce more energy than the one required by the loads, the spare energy can be used either to charge the batteries or to produce H₂ in the electrolyzer. The control strategy optimizes how the spare energy is used. If the amount of energy demanded by the loads is higher than the one produced by the renewable sources, the control strategy determines the most economical way to meet the energy deficit. The optimization of the various system control parameters is done using genetic algorithms. This paper explains the strategy developed and shows its application to a PV–diesel–battery–hydrogen system.     

Beyond smart grids - The need of intelligent energy networks for a higher global efficiency through energy vectors integration

From the energy point of view, the season we have been living more and more seems the era of sources diversification. The most correct scenario for a sustainable energy future foresees no predominance of one source over the others in any area of the world but a proper energy mix, based on locally available resources and needs.The concept and role of energy vectors is key: “(an energy vector) allows transfer, in space and time, a given quantity of energy, hence making it available for use distantly in time and space from the point of availability of the original source”. Therefore, a scenario that gives full scope for the ES to be immune from the characteristics of non-sustainable resource consumption and waste production, the system itself must shift the focus from resources to vectors (mainly electricity, hydrogen, heat).Smart grids have been largely indicate, in this context, as an answer for their characteristic “needful design” to move from the “old” grids (unidirectional power flows) to “new” bi-directional electricity networks. This is not enough: the real need is for an intelligent management of a complete set of energy sources and vectors, as electricity, heat, hydrogen, bio and non-biofuels, that requires a clear shift that goes beyond smart grids and looks at Intelligent Energy Networks.     

Evaluation of log‐of‐power extremum seeking control for wind turbines using large eddy simulations

The extremum seeking control (ESC) algorithm has been proposed to determine operating parameters that maximize power production below rated wind speeds (region II). This is usually done by measuring the turbine's power signal to determine optimal values for parameters of the control law or actuator settings. This paper shows that the standard ESC with power feedback is quite sensitive to variations in mean wind speed, with long convergence time at low wind speeds and aggressive transient response, possibly unstable, at high wind speeds. The paper also evaluates the performance, as measured by the dynamic and steady state response, of the ESC with feedback of the logarithm of the power signal (LP‐ESC). Large eddy simulations (LES) demonstrate that the LP‐ESC, calibrated at a given wind speed, exhibits consistent robust performance at all wind speeds in a typical region II. The LP‐ESC is able to achieve the optimal set‐point within a prescribed settling time, despite variations in the mean wind speed, turbulence, and shear. The LES have been conducted using realistic wind input profiles with shear and turbulence. The ESC and LP‐ESC are implemented in the LES without assuming the availability of analytical gradients.     

Effect of different control strategies on rapid cold start-up of a 30-cell proton exchange membrane fuel cell stack

Many places experience extreme temperatures below −30 °C, which is a great challenge for the fuel cell vehicle (FCV). The aim of this study is to optimize the strategy to achieve rapid cold start-up of the 30-cell stack at different temperature conditions. The test shows that the stack rapidly starts within 30 s at an ambient temperature of −20 °C. Turning on the coolant at −25 °C show stability of the cell voltage at both ends due to the end-plate heating, however, voltage of intermediate cells fluctuates sharply, and successful start-up is completed after 60 s. The cold start strategy changes to load-voltage cooperative control mode when the ambient temperature reduced to −30 °C, the voltage of multiple cells in the middle of the stack fluctuate more drastic, and start-up takes 113 s. The performance and consistency of the stack did not decay after 20 cold start-up experiments, which indicates that our control strategies effectively avoided irreversible damage to the stack caused by freeze-thaw process.     

Comparative study of two new energy control systems based on PEMFC for a hybrid tramway in Ecuador

This article presents a comparison of two alternative systems to supply the traction power of a tramway in Cuenca–Ecuador. Each system studies the effective combination of supercapacitors, lithium ion batteries and proton exchange membrane fuel cells (SC/LIB/PEMFC) on board. The first system uses renewable sources (PV/HKT/GB/Grid) supplying the on-board systems through the existing grid and hydrogen charging stations. While the second system uses only grid power from a single point of charge, leaving the tramway without any external connection point throughout their journey. The energy and economic analyses are based on the capacity of each system to supply the load and the resources used.The results show that the new proposed control systems, by means of the analysed configurations made up of different control states, are always capable of perfectly supplying the power required by the tramway throughout their journey. However, when using energy from renewable sources, hydrogen consumption decreases by 4.27% with respect to the grid on each round trip, with a lower net present cost. Furthermore, in the first proposed system, the depth of discharge in SC and LIB is greater.     

Architecture and performance of neural networks for efficient A/C control in buildings

The feasibility of using neural networks (NNs) for optimizing air conditioning (AC) setback scheduling in public buildings was investigated. The main focus is on optimizing the network architecture in order to achieve best performance.

To save energy, the temperature inside public buildings is allowed to rise after business hours by setting back the thermostat. The objective is to predict the time of the end of thermostat setback (EoS) such that the design temperature inside the building is restored in time for the start of business hours.

State of the art building simulation software, ESP-r, was used to generate a database that covered the years 1995–1999. The software was used to calculate the EoS for two office buildings using the climate records in Kuwait. The EoS data for 1995 and 1996 were used for training and testing the NNs. The robustness of the trained NN was tested by applying them to a “production” data set (1997–1999), which the networks have never “seen” before.

For each of the six different NN architectures evaluated, parametric studies were performed to determine the network parameters that best predict the EoS. External hourly temperature readings were used as network inputs, and the thermostat end of setback (EoS) is the output. The NN predictions were improved by developing a neural control scheme (NC). This scheme is based on using the temperature readings as they become available. For each NN architecture considered, six NNs were designed and trained for this purpose. The performance of the NN analysis was evaluated using a statistical indicator (the coefficient of multiple determination) and by statistical analysis of the error patterns, including ANOVA (analysis of variance).

The results show that the NC, when used with a properly designed NN, is a powerful instrument for optimizing AC setback scheduling based only on external temperature records.     

Online performance evaluation of alternative control strategies for building cooling water systems prior to in situ implementation

This paper presents the online test and evaluation of the performance of five practical control strategies (fixed set-point control method, fixed approach control method, two near optimal strategies and one optimal strategy) for building cooling water systems to identify the best strategy for future field validation. All of these strategies were tested and evaluated in a simulated virtual environment similar to the situation when they are actually implemented in practice. A virtual building system representing the real building and its central chilling system was developed and used to test the operational performance of the system controlled by different strategies. The packages of each control strategy are separately computed by the application program of Matlab, as the control optimizers to identify the necessary control settings for the given condition based on the collected operation data. The data exchanger between the virtual building system and the control optimizer was managed by a software platform through a communication interface. The results showed that the optimal control strategy is more energy efficient and cost effective than the other strategies, and its computational cost is manageable and can satisfy the requirements of practical applications. This strategy is being implemented in a super high-rise building for field validation.     

Comparison of methodologies for tmy generation using 20 years data for Athens, Greece

The need of accurate Test Reference Years (TRYs) for simulations has been well recognised over the years. Various methods for deriving TRYs have been developed, but their final results can be significantly different. In this paper, the major methodologies reported in literature were applied to 20-year hourly measurements of weather data from Athens, covering the period 1977 to 1996. Seventeen TRYs were produced in total. The basis to select the “best” performing TRY includes meteorological criteria (inherent in the selection process used by each method) and comparisons of results from various simulations for typical energy systems (i.e. a solar water heater, a building, a large scale solar heating system with interseasonal storage and a photovoltaic system). Based on the results of each simulation exercise, a scoring system was developed and applied. The best performing TRY was found to be the one produced by a modified Festa-Ratto method.     

Energy saving by integrated control of natural ventilation and HVAC systems using model guide for comparison

Integrated control by controlling both natural ventilation and HVAC systems based on human thermal comfort requirement can result in significant energy savings. The concept of this paper differs from conventional methods of energy saving in HVAC systems by integrating the control of both these HVAC systems and the available natural ventilation that is based on the temperature difference between the indoor and the outdoor air. This difference affects the rate of change of indoor air enthalpy or indoor air potential energy storage. However, this is not efficient enough as there are other factors affecting the rate of change of indoor air enthalpy that should be considered to achieve maximum energy saving. One way of improvement can be through the use of model guide for comparison (MGFC) that uses physical-empirical hybrid modelling to predict the rate of change of indoor air potential energy storage considering building fabric and its fixture. Three methods (normal, conventional and proposed) are tested on an identical residential building model using predicted mean vote (PMV) sensor as a criterion test for thermal comfort standard. The results indicate that the proposed method achieved significant energy savings compared with the other methods while still achieving thermal comfort.     

Comparison of three different control strategies for load-change and attenuation of solid oxide fuel cell system

Solid oxide fuel cell (SOFC) with a lot of advantages, such as high efficiency, low emission and great fuel compatibility, has broad application prospects in many fields. However, an appropriate control strategy is necessary for SOFC systems, which could not only maintain high system efficiency during load-change, but also supplement power after attenuation to extend system service life. In the article, three different control strategies are proposed, in which fuel flow, fuel utilization and cell voltage are controlled as constants respectively. The performance and applicability of strategies for load-change and cell degradation are evaluated through experiment data and simulations. Meanwhile, stack temperature, voltage, fuel utilization and efficiency are selected as main constraints to analyze the application scope of strategies. And in load increasing process of a 1 kW SOFC combined heat and power (CHP) system fed with methanol, the strategies are adopted to verify their effectiveness.     

A comparative study of three different sensorless vector control strategies for a Flywheel Energy Storage System

The aim of this paper is to ensure the sensorless control of an inertial storage system associated to an isolated Hybrid Energy Production Unit (HEPU). The Flywheel Energy Storage System (FESS) is used as energy buffers in order to store or retrieve energy into a stand-alone load. A comparative study of three different techniques based on a sensorless vector-controlled induction motor (IM) driving a flywheel are presented. First, a speed estimation algorithm based on model reference adaptive system (MRAS) theory is proposed. Then, a model reference adaptive speed observers is introduced in this paper with an accurate stability study. This observer strategy is then ameliorated with a new reduced adaptive speed observer. The observer parameters are adapted during flux weakening in order to obtain close tracking of the flywheel speed. The accuracy of the presented models is confirmed by simulation results.     

Comparison of three different solar collectors integrated with geothermal source for electricity and hydrogen production

In this study, an integrated system is proposed for mainly electricity and hydrogen production. Energy and exergy analyses of the system are also examined by using Engineering Equation Solver (EES, version 2019) under solar radiation during day time on 1st July. The proposed system consists of a middle-temperature geothermal source with fluid temperature 93 °C, three solar collectors (SCs of 300 m²) namely parabolic trough solar collectors (PTSCs), evacuated tube solar collectors (ETSCs), flat plate solar collectors (FPSCs), an organic Rankine cycle (ORC), proton exchange membrane (PEM), a compressor, hot water storage tank and a mushroom cultivation room. The temperature of the geothermal fluid is upgraded via solar collectors by harvesting solar radiation to operate the ORC. Thus the generated electricity is used in the PEM electrolysis system for producing hydrogen. When the PTSCs, ETSCs, and FPSCs are integrated with the geothermal source separately, it is found that 2758.69 g, 1585.27 g, and 634.42 g of hydrogen can be produced, respectively for a day. The highest overall energetic and exergetic performance of the system is calculated as to be 5.67% and 7.49%, respectively.     

Dynamics and control of a thermally self-sustaining energy storage system using integrated solid oxide cells for an islanded building

A solid oxide cell-based energy system is proposed for a solar-powered stand-alone building. The system is comprised of a 5 kW_el solid oxide fuel cell (SOFC), a 9.5 kW_el solid oxide electrolysis cell (SOEC), and the required balance of plant. The SOFC supplies: 1- building demand in the absence of sufficient solar power, 2- heat for SOEC in endothermic and standby modes. Thermal integration of SOFC and SOEC is implemented through a network of heat exchangers, combined with set of control algorithms. Two control strategies were implemented to actuate the SOFC in response to endothermic heat demands of SOEC by manipulating: 1- electric power, 2- fuel utilization. The results of dynamic simulation of system for two scenarios (sunny day and cloudy day) showed successful compliance of temperature constraints with both methods. Manipulation of fuel utilization, however, resulted in better system performance in terms of efficiency and H₂ balance.