Murray Thomson

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.

A small-scale seawater reverse-osmosis system with excellent energy efficiency over a wide operating range

A small-scale seawater reverse-osmosis system with excellent energy efficiency is presented. The system promises to deliver up to 460 l/h of potable water, from seawater (at 40,000 ppm), while consuming less than 1600 W of electrical power. This represents a specific energy consumption of less than 3.5 kWh/m3. Moreover, the flow may be controlled in order to reduce the power consumption by a factor of four without any significant loss of efficiency — the specific energy consumption remains near to 3.5 kWh/m3. The keys to these impressive figures are the energy recovery provided by the Clark pump, from Spectra Watermakers Inc., and the use of a variable water recovery ratio control algorithm, developed by CREST. The significance of the system is that it may be operated from variable intermittent renewable-energy sources, such as wind and solar-photovoltaic (PV), without need of batteries. Results of laboratory testing and extensive modeling are presented.

Assessing heat pumps as flexible load

In a future power system featuring significant renewable generation, the ability to manipulate domestic demand through the flexible operation of heat-led technologies such as heat pumps and micro-combined heat and power could be a critical factor in providing a secure and stable supply of electrical energy. Using a simulation-based approach, this study examined the linkage between the thermal characteristics of buildings and the scope for flexibility in the operating times of air source heat pumps. This was assessed against the resulting impact on the end-user’s comfort and convenience. A detached dwelling and flat were modelled in detail along with their heating system in order to determine the temporal shift achievable in the heat pump operating times for present-day and future dwellings. The simulation results indicated that the scope for shifting heat pump operating times in the existing building stock was limited, with time shifts of only 1–2 h achieved before there was a serious impact on the comfort of the occupant. However, if insulation levels were dramatically improved and substantial levels of thermal buffering were added into the heating system, sizable time shifts of up to 6 h were achievable without a significant impact on either space or hot water temperatures.

“For the times they are a-changin”: the impact of shifting energy-use practices in time and space

The introduction of extensive wind power in pursuit of 2050 carbon reduction targets presents a major challenge to electricity networks because when the wind blows (supply) does not necessarily match when people want to use electricity (demand). As electricity storage remains very expensive, flexible demand will have an important role in balancing the grid. While there is scope for smart solutions such as automation and pricing, people will need to become more flexible in the longer term. Accordingly, the aim of this research was to look at time-shifting energy use. Using practice theory helped move the study beyond a merely technical, individualised or structural approach. This interdisciplinary research used 24-hour in-house observations, interviews, metered energy data and three energy time-shifting challenges. The results challenge current approaches to demand response and suggest that disruption is a normal part of everyday life around which practices are able to rearrange themselves and that it is, therefore, possible to consider changing energy-use practices. While it is necessary to consider the relationships between practices and the fact that they are temporally and spatially anchored, it is possible to locate agency within them and therefore to suggest strategies for changing them. Unlocking this flexibility remains the challenge but a range of innovative options for doing this is suggested.

Integrated simulation of photovoltaic micro-generation and domestic electricity demand: a one-minute resolution open-source model

Domestic photovoltaic generation can partially offset the electricity demand within an individual dwelling. The net demand may be readily estimated on an annual basis, but modelling its import and export with respect to time, is more complex. A key issue is that domestic electricity demand, particularly lighting, is significantly influenced by the outdoor light level, which of course also has a direct effect on photovoltaic generation. Thus, realistic time-step simulation of the net demand requires that the two components are modelled with respect to a common representation of the solar irradiance. This article presents the construction of an integrated model that provides data at a one-minute time resolution, built upon a fully validated high-resolution electricity demand model. An open-source software implementation of the integrated model in VBA within Microsoft Excel is described and is available for free download.

Network power-flow analysis for a high penetration of distributed generation

Summary form only given. The connection of large numbers of small or micro-generators to low voltage distribution networks may put at risk traditional network designs. Such designs are based on traditional assumptions such as historical diversity factors and other simplifying assumptions that may no longer be appropriate. This paper describes on-going research, based on a detailed examination of a typical low voltage distribution system with increasing penetrations of distributed generation, mostly in the form of micro-generators (micro-clip, building integrated wind, and building integrated photovoltaics) connected at domestic properties. Modelling techniques have been developed that are applicable to complete distribution feeders: from primary substations, through medium and low-voltage networks to individual single-phase customer connection points. The modelling uses accurate unbalanced power-flow analysis (load-flow) and 1-minute time-series load data. Provisional results show that statistically, in terms of voltage quality and line limits, higher than expected penetrations of distributed generation can be accommodated without modifications to the distribution system. System evolution is discussed with even higher penetrations in mind.

A modelling framework for the study of highly distributed power systems and demand side management

This paper presents an approach to the modelling and simulation of highly distributed power systems, in the context of existing UK distribution networks. The model is part of a framework to assess the network impact of high-penetrations of micro-generation, as well as demand side management (DSM) measures. This framework supports the representation of the 11 kV and low voltage distribution network within a geospatial model, together with an integrated load-flow computational capability. A domestic electricity demand model is incorporated within the framework that generates synthetic demand profiles within the network. The framework is used to investigate the impact of distributed generation for a case study of a regional UK town, together with the surrounding villages and rural locale. The scope includes primary distribution substations, 11 kV feeders and a low-voltage distribution network, serving approximately 35 000 domestic properties and commercial premises. This case study also includes the measurement of minute-by-minute residential electricity usage within a number of individual domestic houses and distribution sub-stations.

Modelling the impact of micro-combined heat and power generators on electricity distribution networks

This paper investigates potential technical effects that a high take up of domestic micro-CHP could have on an electricity distribution system. This study is based on a combination of house-by-house energy use modelling and network power-flow analysis. A variety of micro-CHP technologies are represented, including Stirling engines, internal combustion engines, and fuel cells. These have different heat-to-power ratios and thus different impacts on the electricity system. The results and discussion focus on voltage rise, which is considered to be the primary constraint on allowable penetration.

Diagramming social practice theory: An interdisciplinary experiment exploring practices as networks

Achieving a transition to a low-carbon energy system is now widely recognised as a key challenge facing humanity. To date, the vast majority of research addressing this challenge has been conducted within the disciplines of science, engineering and economics utilising quantitative and modelling techniques. However, there is growing awareness that meeting energy challenges requires fundamentally sociotechnical solutions and that the social sciences have an important role to play. This is an interdisciplinary challenge but, to date, there remain very few explorations of, or reflections on, interdisciplinary energy research in practice. This paper seeks to change that by reporting on an interdisciplinary experiment to build new models of energy demand on the basis of cutting-edge social science understandings. The process encouraged the social scientists to communicate their ideas more simply, whilst allowing engineers to think critically about the embedded assumptions in their models in relation to society and social change. To do this, the paper uses a particular set of theoretical approaches to energy use behaviour known collectively as social practice theory – and explores the potential of more quantitative forms of network analysis to provide a formal framework by means of which to diagram and visualise practices. The aim of this is to gain insight into the relationships between the elements of a practice, so increasing the ultimate understanding of how practices operate. Graphs of practice networks are populated based on new empirical data drawn from a survey of different types (or variants) of laundry practice. The resulting practice networks are analysed to reveal characteristics of elements and variants of practice, such as which elements could be considered core to the practice, or how elements between variants overlap, or can be shared. This promises insights into energy intensity, flexibility and the rootedness of practices (i.e. how entrenched/established they are) and so opens up new questions and possibilities for intervention. The novelty of this approach is that it allows practice data to be represented graphically using a quantitative format without being overly reductive. Its usefulness is that it is readily applied to large datasets, provides the capacity to interpret social practices in new ways and serves to open up potential links with energy modelling. More broadly, a significant dimension of novelty has been the interdisciplinary approach, radically different to that normally seen in energy research. This paper is relevant to a broad audience of social scientists and engineers interested in integrating social practices with energy engineering.

Wind Resource Assessment for the Kingdom of Bahrain

The geographical distribution of wind speed (the wind atlas) for the kingdom of Bahrain is presented, based on measured data and on calculations undertaken using WAsP,. The data used were recorded by the Meteorological Directorate at a weather station situated at Bahrain International Airport, taken on an hourly basis for a period of time extended for ten years. These data indicate an annual mean wind speed of 4.6 m/s at 10 m height and mean Weibull scale and shape parameters C and k of 5.2 m/s and 1.9 respectively. At a typical wind turbine hub height of sixty metres, these values are extrapolated to 6.9 m/s, 7.8 m/s and 1.8 respectively, which suggests that the area has a good wind resource. The wind atlas shows that several locations in the less populated central and southern regions of the main island of the archipelago of Bahrain are potentially suitable for wind energy production.