Structural Analysis and Control of a Model of Two-site Electricity and Heat Supply

This paper introduces a control problem of regulation of energy flows in a two-site electricity and heat supply system, where two Combined Heat and Power (CHP) plants are interconnected via electricity and heat flows. The control problem is motivated by recent development of fast operation of CHP plants to provide ancillary services of power system on the order of tens of seconds to minutes. Due to the physical constraint that the responses of the heat subsystem are not necessary as fast as those of the electric subsystem, the target controlled state is not represented by any isolated equilibrium point, implying that stability of the system is lost in the long-term sense on the order of hours. In this paper, we first prove in the context of nonlinear control theory that the state-space model of the two-site system is non-minimum phase due to nonexistence of isolated equilibrium points of the associated zero dynamics.Instead, we locate a one-dimensional invariant manifold that represents the target controlled flows completely. Then, by utilizing a virtual output under which the state-space model becomes minimum phase, we synthesize a controller that achieves not only the regulation of energy flows in the short-term regime but also stabilization of an equilibrium point in the long-term regime. Effectiveness of the synthesized controller is established with numerical simulations with a practical set of model parameters

Multiobjective Robust Control with HIFOO 2.0

Multiobjective control design is known to be a difficult problem both in theory and practice. Our approach is to search for locally optimal solutions of a nonsmooth optimization problem that is built to incorporate minimization objectives and constraints for multiple plants. We report on the success of this approach using our public-domain Matlab toolbox HIFOO 2.0, comparing our results with benchmarks in the literature

Improvement of Energy Efficiency for Wastewater Treatment

Wastewater treatment requires the elimination of pathogens and reduction of organic matter in the treated sludge to acceptable levels. One process used to achieve this is Autothermal Thermophylic Aerobic Digestion (ATAD), which relies on promoting non-pathogenic thermophilic bacteria to digest organic matter and kill pathogens through metabolic heat generation. This process requires continuous aeration that may be energy consuming, and the final aim of the study is to identify how the process design can minimize the energy input per mass of treated sludge. Appropriate modeling of the reactor process is an essential ingredient, so we explore properties of an existing model and propose a simplified alternative model

Finite-time behavior of inner systems

In this paper, we investigate how nonminimum phase characteristics of a dynamical system affect its controllability and tracking properties. For the class of linear time-invariant dynamical systems, these characteristics are determined by transmission zeros of the inner factor of the system transfer function. The relation between nonminimum phase zeros and Hankel singular values of inner systems is studied and it is shown how the singular value structure of a suitably defined operator provides relevant insight about system invertibility and achievable tracking performance. The results are used to solve various tracking problems both on finite as well as on infinite time horizons. A typical receding horizon control scheme is considered and new conditions are derived to guarantee stabilizability of a receding horizon controller

Effects of composting manures and other organic wastes on soil processes and pest and disease interactions

Introduction Composts and manures are of major importance in providing fertility in organic farming systems, since synthetic fertilisers are prohibited. It is understood that composts have radically different nutrient release characteristics to those of uncomposted materials and manures, and it is believed that composting increases the beneficial effects of organic materials on soil health, soil quality, soil fertility and nutrient use efficiency. It has also been shown that some plant pests and diseases are suppressed through the application of composts and compost extracts to soils. There is considerable potential to use a wider range of feedstocks from on and off-farm sources and to improve the composting process and compost/manure application techniques. This review of scientific work to date was urgently required to help determine key research priorities to achieve this potential (Defra project OF0313). Project aims 1.To document the current standards, regulations and legislation relevant to recycling, compost/manure preparation and application and to review common UK practices relating to the preparation and application of uncomposted materials, manures, composts and compost extracts. 2. To review current scientific knowledge (from the literature) of the effects of different composting processes on chemical and biological parameters in the finished compost or compost extract. 3. To review (from the literature) the effects of uncomposted materials, manures and composts on soil health and quality, soil fertility and crop development and nutrition. 4. To review (from the literature) the effects of uncomposted materials, manures, composts and compost extracts on pest and disease incidence and severity in agricultural and horticultural crops. 5. To outline a proposed strategy for research which seeks to develop composting systems and compost/manure application protocols with a view to optimising soil fertility management and pest and disease control in organic agriculture and horticulture. Objective 1 - The current standards, regulations and legislation relevant to recycling, compost/manure preparation and application are documented in detail in the full report on Objective 1 (Appendix 2). Manures and uncomposted plant materials (e.g. green manures) are commonly used on UK organic farms. True composts (defined in the glossary, Appendix 1) are rarely prepared on UK organic farms, although there is increasing interest in their use, particularly on farms producing high value horticultural crops. An increasing number of companies are producing (or are interested in producing) composts suitable for use on organic farms as soil amendments or growing media. Objective 2 - The effects of different composting processes on chemical and biological parameters in the finished compost or compost extract are reviewed in detail in the full report on Objective 2 (Appendix 3). A short version of this review appears on pages 7-10 of this report. Objective 3 - The effects of uncomposted materials, manures and composts on soil health and quality, soil fertility and crop development and nutrition are reviewed in detail in the full report on Objective 3 (Appendix 4). A short version of this review appears on pages 10-13 of this report. Objective 4 - The effects of uncomposted materials, manures, composts and compost extracts on pest and disease incidence and severity in agricultural and horticultural crops are reviewed in detail in the full report on Objective 4 (Appendix 5). A short version of this review appears on pages 13-17 of this report. Objective 5 - A proposed strategy for research was outlined which seeks to develop composting systems and compost/manure application protocols with a view to optimising soil fertility management and pest/disease control in organic agriculture/horticulture Organic farming systems are by nature holistic. In other words, they function as a whole and all aspects of the system are interdependent on many other aspects of the system. It is essential therefore that research which is carried out to optimise the use of uncomposted plant residues, composts, manures and compost extracts is interdisciplinary; that is it must be carried out with reference to the organic farming system as a whole and not just a single aspect of it. Technology transfer and knowledge transfer are key elements to the proposed strategy for research. Seminars and conferences, farm walks, demonstration farms and a wide range of publishing formats must be used to ensure that end users have full access to the results of research carried out in the UK and abroad. The amount of information which is available for dissemination to those who wish to make or use composts will naturally depend on the amount of relevant research and development work which is going on in the UK, Europe and worldwide

Dual Rate Control for Security in Cyber-physical Systems

We consider malicious attacks on actuators and sensors of a feedback system which can be modeled as additive, possibly unbounded, disturbances at the digital (cyber) part of the feedback loop. We precisely characterize the role of the unstable poles and zeros of the system in the ability to detect stealthy attacks in the context of the sampled data implementation of the controller in feedback with the continuous (physical) plant. We show that, if there is a single sensor that is guaranteed to be secure and the plant is observable from that sensor, then there exist a class of multirate sampled data controllers that ensure that all attacks remain detectable. These dual rate controllers are sampling the output faster than the zero order hold rate that operates on the control input and as such, they can even provide better nominal performance than single rate, at the price of higher sampling of the continuous output