Economic efficiency of mobile latent heat storages

In a pilot project an optimized mobile latent heat storage based on a system available on the market has been tested at Fraunhofer Institute for Environmental, Safety and Energy Technology. Initially trials were conducted with the aim of optimizing the process of charging and discharging. A specifically constructed test rig at the incineration trials centre at the institute allowed charging and discharging procedures of the mobile latent heat storage with adjustable parameters. In addition an evaluation model was constructed to further optimize the heat exchanger systems. In conclusion the prototype of the mobile latent heat storage was tested in practical operation. The economic and technical feasibility of heat transportation was shown if not utilized waste heat is available. © 2014 The Authors.

Analysis and design of power management scheme for an on-board solar energy storage system

This paper investigates the power management issues in a mobile solar energy storage system. A multi-converter based energy storage system is proposed, in which solar power is the primary source while the grid or the diesel generator is selected as the secondary source. The existence of the secondary source facilitates the battery state of charge detection by providing a constant battery charging current. Converter modeling, multi-converter control system design, digital implementation and experimental verification are introduced and discussed in details. The prototype experiment indicates that the converter system can provide a constant charging current during solar converter maximum power tracking operation, especially during large solar power output variation, which proves the feasibility of the proposed design. © 2014 IEEE.

A techno-economic assessment of the use of fast pyrolysis bio-oil from UK energy crops in the production of electricity and combined heat and power

This thesis investigates the cost of electricity generation using bio-oil produced by the fast pyrolysis of UK energy crops. The study covers cost from the farm to the generator’s terminals. The use of short rotation coppice willow and miscanthus as feedstocks was investigated. All costs and performance data have been taken from published papers, reports or web sites. Generation technologies are compared at scales where they have proved economic burning other fuels, rather than at a given size. A pyrolysis yield model was developed for a bubbling fluidised bed fast pyrolysis reactor from published data to predict bio-oil yields and pyrolysis plant energy demands. Generation using diesel engines, gas turbines in open and combined cycle (CCGT) operation and steam cycle plants was considered. The use of bio-oil storage to allow the pyrolysis and generation plants to operate independently of each other was investigated. The option of using diesel generators and open cycle gas turbines for combined heat and power was examined. The possible cost reductions that could be expected through learning if the technology is widely implemented were considered. It was found that none of the systems analysed would be viable without subsidy, but with the current Renewable Obligation Scheme CCGT plants in the 200 to 350 MWe range, super-critical coal fired boilers co-fired with bio-oil, and groups of diesel engine based CHP schemes supplied by a central pyrolysis plant would be viable. It was found that the cost would reduce with implementation and the planting of more energy crops but some subsidy would still be needed to make the plants viable.

Desalination of brackish water by a batch reverse osmosis desalink system for use with solar thermal energy

For remote, semi-arid areas, brackish groundwater (BW) desalination powered by solar energy may serve as the most technically and economically viable means to alleviate the water stresses. For such systems, high recovery ratio is desired because of the technical and economical difficulties of concentrate management. It has been demonstrated that the current, conventional solar reverse osmosis (RO) desalination can be improved by 40–200 times by eliminating unnecessary energy losses. In this work, a batch-RO system that can be powered by a thermal Rankine cycle has been developed. By directly recycling high pressure concentrates and by using a linkage connection to provide increasing feed pressures, the batch-RO has been shown to achieve a 70% saving in energy consumption compared to a continuous single-stage RO system. Theoretical investigations on the mass transfer phenomena, including dispersion and concentration polarization, have been carried out to complement and to guide experimental efforts. The performance evaluation of the batch-RO system, named DesaLink, has been based on extensive experimental tests performed upon it. Operating DesaLink using compressed air as power supply under laboratory conditions, a freshwater production of approximately 300 litres per day was recorded with a concentration of around 350 ppm, whilst the feed water had a concentration range of 2500–4500 ppm; the corresponding linkage efficiency was around 40%. In the computational aspect, simulation models have been developed and validated for each of the subsystems of DesaLink, upon which an integrated model has been realised for the whole system. The models, both the subsystem ones and the integrated one, have been demonstrated to predict accurately the system performance under specific operational conditions. A simulation case study has been performed using the developed model. Simulation results indicate that the system can be expected to achieve a water production of 200 m3 per year by using a widely available evacuated tube solar collector having an area of only 2 m2. This freshwater production would satisfy the drinking water needs of 163 habitants in the Rajasthan region, the area for which the case study was performed.

Second life battery energy storage systems:converter topology and redundancy selection

Battery energy storage systems have traditionally been manufactured using new batteries with a good reliability. The high cost of such a system has led to investigations of using second life transportation batteries to provide an alternative energy storage capability. However, the reliability and performance of these batteries is unclear and multi-modular power electronics with redundancy have been suggested as a means of helping with this issue. This paper reviews work already undertaken on battery failure rate to suggest suitable figures for use in reliability calculations. The paper then uses reliability analysis and a numerical example to investigate six different multi-modular topologies and suggests how the number of series battery strings and power electronic module redundancy should be determined for the lowest hardware cost using a numerical example. The results reveal that the cascaded dc-side modular with single inverter is the lowest cost solution for a range of battery failure rates.

Analysis and comparative study of different converter modes in modular second life hybrid battery energy storage systems

The use of ex-transportation battery system (i.e. second life EV/HEV batteries) in grid applications is an emerging field of study. A hybrid battery scheme offers a more practical approach in second life battery energy storage systems because battery modules could be from different sources/ vehicle manufacturers depending on the second life supply chain and have different characteristics e.g. voltage levels, maximum capacity and also different levels of degradations. Recent research studies have suggested a dc-side modular multilevel converter topology to integrate these hybrid batteries to a grid-tie inverter. Depending on the battery module characteristics, the dc-side modular converter can adopt different modes such as boost, buck or boost-buck to suitably transfer the power from battery to the grid. These modes have different switching techniques, control range, different efficiencies, which give a system designer choice on operational mode. This paper presents an analysis and comparative study of all the modes of the converter along with their switching performances in detail to understand the relative advantages and disadvantages of each mode to help to select the suitable converter mode. Detailed study of all the converter modes and thorough experimental results based on a multi-modular converter prototype based on hybrid batteries has been presented to validate the analysis.

Control of second-life hybrid battery energy storage system based on modular boost-multilevel buck converter

To fully utilize second-life batteries on the grid system, a hybrid battery scheme needs to be considered for several reasons: the uncertainty over using a single source supply chain for second-life batteries, the differences in evolving battery chemistry and battery configuration by different suppliers to strive for greater power levels, and the uncertainty of degradation within a second-life battery. Therefore, these hybrid battery systems could have widely different module voltage, capacity, and initial state of charge and state of health. In order to suitably integrate and control these widely different batteries, a suitable multimodular converter topology and an associated control structure are required. This paper addresses these issues proposing a modular boost-multilevel buck converter based topology to integrate these hybrid second-life batteries to a grid-tie inverter. Thereafter, a suitable module-based distributed control architecture is introduced to independently utilize each converter module according to its characteristics. The proposed converter and control architecture are found to be flexible enough to integrate widely different batteries to an inverter dc link. Modeling, analysis, and experimental validation are performed on a single-phase modular hybrid battery energy storage system prototype to understand the operation of the control strategy with different hybrid battery configurations.

Sizing energy storage on the 11kV distribution network

The cost and limited flexibility of traditional approaches to 11kV network reinforcement threatens to constrain the uptake of low carbon technologies. Ofgem has released £500m of funding for DNOs to trial innovative techniques and share the learning with the rest of the industry. One of the techniques under study is the addition of Energy Storage at key substations to the network to help with peak load lopping. This paper looks in detail at the sizing algorithm for use in the assessment of alternatives to traditional reinforcement and investigates a method of sizing a battery for use on a Network taking into account load growth, capacity fade and battery lifecycle issues. A further complication to the analysis is the method of operation of the battery system and how this affects the Depth of Discharge (DoD). The proposed method is being trialled on an area of 11kV network in Milton Keynes Central area and the simulation results are presented in this paper.

A multi-modular second life hybrid battery energy storage system for utility grid applications

The modern grid system or the smart grid is likely to be populated with multiple distributed energy sources, e.g. wind power, PV power, Plug-in Electric Vehicle (PEV). It will also include a variety of linear and nonlinear loads. The intermittent nature of renewable energies like PV, wind turbine and increased penetration of Electric Vehicle (EV) makes the stable operation of utility grid system challenging. In order to ensure a stable operation of the utility grid system and to support smart grid functionalities such as, fault ride-through, frequency response, reactive power support, and mitigation of power quality issues, an energy storage system (ESS) could play an important role. A fast acting bidirectional energy storage system which can rapidly provide and absorb power and/or VARs for a sufficient time is a potentially valuable tool to support this functionality. Battery energy storage systems (BESS) are one of a range suitable energy storage system because it can provide and absorb power for sufficient time as well as able to respond reasonably fast. Conventional BESS already exist on the grid system are made up primarily of new batteries. The cost of these batteries can be high which makes most BESS an expensive solution. In order to assist moving towards a low carbon economy and to reduce battery cost this work aims to research the opportunities for the re-use of batteries after their primary use in low and ultra-low carbon vehicles (EV/HEV) on the electricity grid system. This research aims to develop a new generation of second life battery energy storage systems (SLBESS) which could interface to the low/medium voltage network to provide necessary grid support in a reliable and in cost-effective manner. The reliability/performance of these batteries is not clear, but is almost certainly worse than a new battery. Manufacturers indicate that a mixture of gradual degradation and sudden failure are both possible and failure mechanisms are likely to be related to how hard the batteries were driven inside the vehicle. There are several figures from a number of sources including the DECC (Department of Energy and Climate Control) and Arup and Cenex reports indicate anything from 70,000 to 2.6 million electric and hybrid vehicles on the road by 2020. Once the vehicle battery has degraded to around 70-80% of its capacity it is considered to be at the end of its first life application. This leaves capacity available for a second life at a much cheaper cost than a new BESS Assuming a battery capability of around 5-18kWhr (MHEV 5kWh - BEV 18kWh battery) and approximate 10 year life span, this equates to a projection of battery storage capability available for second life of >1GWhrs by 2025. Moreover, each vehicle manufacturer has different specifications for battery chemistry, number and arrangement of battery cells, capacity, voltage, size etc. To enable research and investment in this area and to maximize the remaining life of these batteries, one of the design challenges is to combine these hybrid batteries into a grid-tie converter where their different performance characteristics, and parameter variation can be catered for and a hot swapping mechanism is available so that as a battery ends it second life, it can be replaced without affecting the overall system operation. This integration of either single types of batteries with vastly different performance capability or a hybrid battery system to a grid-tie 3 energy storage system is different to currently existing work on battery energy storage systems (BESS) which deals with a single type of battery with common characteristics. This thesis addresses and solves the power electronic design challenges in integrating second life hybrid batteries into a grid-tie energy storage unit for the first time. This study details a suitable multi-modular power electronic converter and its various switching strategies which can integrate widely different batteries to a grid-tie inverter irrespective of their characteristics, voltage levels and reliability. The proposed converter provides a high efficiency, enhanced control flexibility and has the capability to operate in different operational modes from the input to output. Designing an appropriate control system for this kind of hybrid battery storage system is also important because of the variation of battery types, differences in characteristics and different levels of degradations. This thesis proposes a generalised distributed power sharing strategy based on weighting function aims to optimally use a set of hybrid batteries according to their relative characteristics while providing the necessary grid support by distributing the power between the batteries. The strategy is adaptive in nature and varies as the individual battery characteristics change in real time as a result of degradation for example. A suitable bidirectional distributed control strategy or a module independent control technique has been developed corresponding to each mode of operation of the proposed modular converter. Stability is an important consideration in control of all power converters and as such this thesis investigates the control stability of the multi-modular converter in detailed. Many controllers use PI/PID based techniques with fixed control parameters. However, this is not found to be suitable from a stability point-of-view. Issues of control stability using this controller type under one of the operating modes has led to the development of an alternative adaptive and nonlinear Lyapunov based control for the modular power converter. Finally, a detailed simulation and experimental validation of the proposed power converter operation, power sharing strategy, proposed control structures and control stability issue have been undertaken using a grid connected laboratory based multi-modular hybrid battery energy storage system prototype. The experimental validation has demonstrated the feasibility of this new energy storage system operation for use in future grid applications.

Synthetic strategies to nanostructured photocatalysts for CO2 reduction to solar fuels and chemicals

Artificial photosynthesis represents one of the great scientific challenges of the 21st century, offering the possibility of clean energy through water photolysis and renewable chemicals through CO2 utilisation as a sustainable feedstock. Catalysis will undoubtedly play a key role in delivering technologies able to meet these goals, mediating solar energy via excited generate charge carriers to selectively activate molecular bonds under ambient conditions. This review describes recent synthetic approaches adopted to engineer nanostructured photocatalytic materials for efficient light harnessing, charge separation and the photoreduction of CO2 to higher hydrocarbons such as methane, methanol and even olefins.

Adaptive control of hybrid battery energy storage systems under capacity fade

There is an increasing call for applications which use a mixture of batteries. These hybrid battery solutions may contain different battery types for example; using second life ex-transportation batteries in grid support applications or a combination of high power, low energy and low power, high energy batteries to meet multiple energy requirements or even the same battery types but under different states of health for example, being able to hot swap out a battery when it has failed in an application without changing all the batteries and ending up with batteries with different performances, capacities and impedances. These types of applications typically use multi-modular converters to allow hot swapping to take place without affecting the overall performance of the system. A key element of the control is how the different battery performance characteristics may be taken into account and the how the power is then shared among the different batteries in line with their performance. This paper proposes a control strategy which allows the power in the batteries to be effectively distributed even under capacity fade conditions using adaptive power sharing strategy. This strategy is then validated against a system of three different battery types connected to a multi-modular converter both with and without capacity fade mechanisms in place.

Control of cascaded DC-DC converter based hybrid battery energy storage systems - Part II:Lyapunov approach

A cascaded DC-DC boost converter is one of the ways to integrate hybrid battery types within a grid-tie inverter. Due to the presence of different battery parameters within the system such as, state-of-charge and/or capacity, a module based distributed power sharing strategy may be used. To implement this sharing strategy, the desired control reference for each module voltage/current control loop needs to be dynamically varied according to these battery parameters. This can cause stability problem within the cascaded converters due to relative battery parameter variations when using the conventional PI control approach. This paper proposes a new control method based on Lyapunov Functions to eliminate this issue. The proposed solution provides a global asymptotic stability at a module level avoiding any instability issue due to parameter variations. A detailed analysis and design of the nonlinear control structure are presented under the distributed sharing control. At last thorough experimental investigations are shown to prove the effectiveness of the proposed control under grid-tie conditions.

Control of cascaded DC-DC converter-based hybrid battery energy storage systems - Part I:stability Issue

There is an emerging application which uses a mixture of batteries within an energy storage system. These hybrid battery solutions may contain different battery types. A DC-side cascaded boost converters along with a module based distributed power sharing strategy has been proposed to cope with variations in battery parameters such as, state-of-charge and/or capacity. This power sharing strategy distributes the total power among the different battery modules according to these battery parameters. Each module controller consists of an outer voltage loop with an inner current loop where the desired control reference for each control loop needs to be dynamically varied according to battery parameters to undertake this sharing. As a result, the designed control bandwidth or stability margin of each module control loop may vary in a wide range which can cause a stability problem within the cascaded converter. This paper reports such a unique issue and thoroughly investigates the stability of the modular converter under the distributed sharing scheme. The paper shows that a cascaded PI control loop approach cannot guarantee the system stability throughout the operating conditions. A detailed analysis of the stability issue and the limitations of the conventional approach are highlighted. Finally in-depth experimental results are presented to prove the stability issue using a modular hybrid battery energy storage system prototype under various operating conditions.

Shell-side transfer in shell-and-tube heat exchangers

Local mass transfer coefficients were determined by using the electrochemical technique. A simple model of a heat exchanger with segmental nickel tube joined to p.v.c. rods replaced the exchanger tubes. Measurements were made for both no-Ieakage, semi-leakage and total leakage configurations. Baffle-spacings of 47.6 mm, 66.6 mm, 97 mm and 149.2 mm wer studied. Also studied were the overall exchanger pressure drops for each configuration. The comparison of the heat transfer data with this work showed good agreement at high flow rates for the no-leakage case, but the agreement became poor for lower flow rates and leakage configurations. This disagreement was explained by non-analogous driving forces existing in the two systems. The no-leakage data showed length-wise variation of transfer coefficients along the exchanger length. The end compartments showing transfer coefficients inferior by up to 26% compared to tbe internal compartments, depending on Reynolds number. With the introduction of leakage streams this variation however became smaller than the experimental accuracy. A model is outlined to show the characteristic behaviour of individual electrode segments within the compartment. This was able to discriminate between cross and window zones for the no- leakage case, but no such distinction could be made for the leakage case. A flow area was found which, when incorporated in the Reynolds number, enabled the correlation of baffle-cut and baffle-spacing parameters for the no-leakage case . This area is the free flow area determined at the baffle edge. Addition of the leakage area to this flow area resulted in correlation of all commercial leakage geometrical parameters. The procedures used to correlate the pressure drop data from a total of eighteen different configurations on a single curve are also outlined.