David Infield

Energy storage and its use with intermittent renewable energy

A simple probabilistic method has been developed to predict the ability of energy storage to increase the penetration of intermittent embedded renewable generation (ERG) on weak electricity grids and to enhance the value of the electricity generated by time-shifting delivery to the network. This paper focuses on the connection of wind generators at locations where the level of ERG would be limited by the voltage rise. Short-term storage, covering less than 1 h, offers only a small increase in the amount of electricity that can be absorbed by the network. Storage over periods of up to one day delivers greater energy benefits, but is significantly more expensive. Different feasible electricity storage technologies are compared for their operational suitability over different time scales. The value of storage in relation to power rating and energy capacity has been investigated so as to facilitate appropriate sizing.

Stabilization of Grid Frequency Through Dynamic Demand Control

Frequency stability in electricity networks is essential to the maintenance of supply quality and security. This paper investigates whether a degree of built-in frequency stability could be provided by incorporating dynamic demand control into certain consumer appliances. Such devices would monitor system frequency (a universally available indicator of supply-demand imbalance) and switch the appliance on or off accordingly, striking a compromise between the needs of the appliance and the grid. A simplified computer model of a power grid was created incorporating aggregate generator inertia, governor action and load-frequency dependence plus refrigerators with dynamic demand controllers. Simulation modelling studies were carried out to investigate the systems response to a sudden loss of generation, and to fluctuating wind power. The studies indicated a significant delay in frequency-fall and a reduced dependence on rapidly deployable backup generation.

Design optimization of thermoelectric devices for solar power generation

Abstract We present an improved theoretical model of a thermoelectric device which has been developed for geometrical optimization of the thermoelectric element legs and prediction of the performance of an optimum device in power generation mode. In contrast to the currently available methods, this model takes into account the effect of all the parameters contributing to the heat transfer process associated with the thermoelectric device. The model is used for a comparative evaluation of four thermoelectric modules. One of these is commercially available and the others are assumed to have an optimum geometry but with different design parameters (thermal and electrical contact layer properties). Results from the model are compared with experimental data of the commercial thermoelectric module in power generation mode with temperature gradient consistent with those achievable from a solar concentrator system. These show that it is important to have devices optimized specifically for generation, and to improve the contact layer of the thermoelements accordingly.

Modeling the Benefits of Vehicle-to-Grid Technology to a Power System

Electric vehicle (EV) numbers are expected to significantly increase in the coming years reflecting their potential to reduce air pollutants and greenhouse gas emissions. Charging such vehicles will impose additional demands on the electricity network but given the pattern of vehicle usage, the possibility exists to discharge the stored energy back to the grid when required, for example when lower than expected wind generation is available. Such vehicle-to-grid operation could see vehicle owners supplying the grid if they are rewarded for providing such services. This paper describes a model of an electric vehicle storage system integrated with a standardized power system (the IEEE 30-node power system model). A decision-making strategy is established for the deployment of the battery energy stored, taking account of the state of charge, time of day, electricity prices and vehicle charging requirements. Applying empirical data, the benefits to the network in terms of load balancing and the energy and cost savings available to the vehicle owner are analyzed. The results show that for the case under study, the EVs have only a minor impact on the network in terms of distribution system losses and voltage regulation but more importantly the vehicle owners costs are roughly halved.

Thermal modelling of a building with an integrated ventilated PV façade

This paper presents a dynamic thermal model based on TRNSYS, for a building with an integrated ventilated PV facade/solar air collector system. The building model developed has been validated against experimental data from a 6.5 m high PV facade on the Mataro Library near Barcelona. Preheating of the ventilation air within the facade is through incident solar radiation heating of the PV elements and subsequent heat transmission to the air within the ventilation gap. The warmed air can be used for building heating in winter. Modelled and measured air temperatures are found to be in good agreement. The heating and cooling loads for the building with and without such a ventilated facade have been calculated and the impact of climatic variations on the performance such buildings has also been investigated. It was found that the cooling loads are marginally higher with the PV facade for all locations considered, whereas the impact of the facade on the heating load depends critically on location.

A photovoltaic-powered seawater reverse-osmosis system without batteries

An efficient cost-effective batteryless photovoltaic-powered seawater reverse-osmosis desalination system is described. The system has a modest 2.4 kWp photovoltaic array and yet promises to deliver 3 m3/d throughout the year in an example location in Eritrea, operating from borehole seawater (at 40,000 ppm). Existing demonstrations of photovoltaic-powered desalination generally employ lead-acid batteries, which allow the equipment to operate at constant flow. In practice however, batteries are notoriously problematic, especially in hot climates. The system employed here operates at variable flow, enabling it to make efficient use of the naturally varying solar resource, without need of batteries. The system employs standard industrial inverters, motors and pumps, which offer excellent energy and cost efficiency. Maximum power point tracking (MPPT) for the photovoltaic array is provided by a novel and extremely simple control algorithm, developed by CREST. Performance and cost estimates from laboratory testing and extensive modelling are presented.

Power quality from multiple grid-connected single-phase inverters

This paper reports on a study into the aggregate power quality from multiple grid connected inverters. Measurements are presented for individual single-phase inverters generating into the low voltage network under a range of operational conditions, and for groups of similar converters connected at the same point on the network. Some results from the modeling of multiple inverter interaction are also presented. Particular attention is given to power factor, the harmonic content of the generated currents, and dc injection.

Application of Auto-Regressive Models to U.K. Wind Speed Data for Power System Impact Studies

Scientific research to characterize the long-term wind energy resource is plentiful. However, if the impact of wind power on the electric power system is the goal of modeling, consideration must be given to diurnal and seasonal effects, as well as the correlation of wind speed between geographical areas. This paper provides such detail by modeling these effects explicitly, enabling accurate evaluations of wind power impact on future power systems to be carried out. This is increasingly important in the context of ambitious wind energy targets driven in the U.K., for example, by the requirement for 20% of Europes energy to be met from renewable energy sources by 2020. Both univariate and multivariate auto-regressive models are presented here and it is shown how they can be applied to geographically dispersed wind speed data. These models are applied to suitably de-trended data. The accuracy of the models is assessed both by inspection of the residuals and by assessment of the forecasting accuracy of the models. Finally, it is shown how the models can be used to synthesize wind speed and thus wind power time series with the correct seasonal, diurnal, and spatial diversity characteristics.

A wind-powered seawater reverse-osmosis system without batteries

The development of small-scale stand-alone desalination systems is important to communities on islands and in isolated inland areas. In such places, electricity supplies are often expensive and unreliable, while the wind resource is abundant. The system presented here comprises a 2.2 kW wind turbine generator powering a variable-flow Reverse osmosis (RO) desalination unit. It is highly efficient, rugged, built with off-the-shelf components and suitable for use in remote areas. Operation at variable-flow allows the uncertainty and variability of the wind to be accommodated without need of energy storage. Batteries, which are common in stand-alone systems, are avoided and water production is dependent on the instantaneous wind speed. A model-based control strategy is used to independently maximize both the energy extracted from the wind and the water output of the RO unit. A computer model of the system has been developed based on component models, identified through laboratory testing. Performance predictions are presented and discussed.

Design and thermal analysis of a two stage solar concentrator for combined heat and thermoelectric power generation

Abstract A design procedure and thermal performance analysis of a two stage solar energy concentrator suited to combined heat and thermoelectric power generation are presented. The concentrator is comprised of a primary one axis parabolic trough concentrator and a second stage compound parabolic concentrator mounted at the focus of the primary. The thermoelectric device is attached to the absorber plate at the focus of the secondary. A cooling tube is fitted to the cold side of the thermoelectric device to extract the waste heat and maintain a high temperature gradient across the device to improve conversion efficiency. The key requirements of the concentrator design are to be tolerant of tracking misalignment, maintain temperature gradients to suit thermoelectric generation and minimise heat losses. A design methodology is presented which allows interception of rays within an angular region ±δ . This results in a wider receiver for the parabolic trough concentrator than would usually be used for a similar concentration ratio. The role of the second stage concentrator in limiting heat losses from the absorber plate is evaluated. Results indicate that in addition to improving the concentration efficiency, the second stage compound parabolic concentrator of the proposed design also inhibits convective air movement and, consequently, improves the overall performance of the solar concentrator.