Kasper Vinther

Direct control implementation of a refrigeration system in smart grid

The thermal capacity of the content in a cold storage room renders it possible to shift the governing refrigeration systems power consumption in time, without compromising temperature constraints. In this paper we introduce a method of implementing such a cold storage room into a directly controlled smart grid, by use of a predictive control strategy. In this application the shift in consumption is used to stabilize a small grid by utilizing excess renewable energy to minimize the need for fossil fueled production sources. In order for the centralized grid controller to handle such a node, its flexibility is communicated in form of a simple generic bucket model. Finally, the provided experiments verify the effectiveness of the proposed method.

Evaporator unit as a benchmark for plug and play and fault tolerant control

Abstract This paper presents a challenging industrial benchmark for implementation of control strategies under realistic working conditions. The developed control strategies should perform in a plug & play manner, i.e. adapt to varying working conditions, optimize their performance, and provide fault tolerance. A fault tolerant strategy is needed to deal with a faulty sensor measurement of the evaporation pressure. The design and algorithmic challenges in the control of an evaporator include: unknown model parameters, large parameter variations, varying loads, and external discrete phenomena such as compressor switch on/off or abrupt change in compressor speed.

Absorption cycle heat pump model for control design

Heat pumps have recently received increasing interest due to green energy initiatives and increasing energy prices. In this paper, a nonlinear dynamic model of a single-effect LiBr-water absorption cycle heat pump is derived for simulation and control design purposes. The model is based on an actual heat pump located at a larger district heating plant. The model is implemented in Modelica and is based on energy and mass balances, together with thermodynamic property functions for LiBr and water and staggered grid representations for heat exchangers. Model parameters have been fitted to operational data and different scenarios are simulated to investigate the operational stability of the heat pump. Finally, this paper provides suggestions and examples of derivation of lower order linear models for control design.

Learning-Based Precool Algorithms for Exploiting Foodstuff as Thermal Energy Reserve

Refrigeration is important to sustain high foodstuff quality and lifetime. Keeping the foodstuff within temperature thresholds in supermarkets is also important due to legislative requirements. Failure to do so can result in discarded foodstuff, a penalty fine to the shop owner, and health issues. However, the refrigeration system might not be dimensioned to cope with hot summer days or performance degradation over time. Two learning-based algorithms are therefore proposed for thermostatically controlled loads, which precools the foodstuff in display cases in an anticipatory manner based on how saturated the system has been in recent days. A simulation model of a supermarket refrigeration system is provided and evaluation of the precool strategies shows that negative thermal energy can be stored in foodstuff to cope with saturation. A system model or additional hardware is not required, which makes the algorithms easy to implement in existing systems.

Single temperature sensor based evaporator filling control using excitation signal harmonics

An important aspect of efficient and safe operation of refrigeration and air conditioning systems is superheat control for evaporators. This is conventionally controlled with a pressure sensor, a temperature sensor, an expansion valve and Proportional-Integral (PI) controllers or more advanced model based control. In this paper we show that superheat can be controlled without a pressure sensor and without a model of the system. This is achieved by continuous excitation of the system and by applying Fourier analysis, which gives an error signal that can be used together with standard PI control. The proposed method works for systems with “S” shaped input/output maps that satisfy sigmoid function properties and such behavior has been identified in both an air conditioning and a refrigeration system. Tests on these systems show that the superheat can be controlled to a low level over a large operating range with only one sensor. It is believed that the method in general is applicable to a wide variety of nonlinear systems for which the desired operating points are close to points of zero mean curvature of system nonlinearities.

Analysis of decentralized control for absorption cycle heat pumps

This paper investigates decentralized control structures for absorption cycle heat pumps and a dynamic nonlinear model of a single-effect LiBr-water absorption system is used as case study. The model has four controllable inputs, which can be used to stabilize the operation of the heat pump under different load conditions. Different feasible input-output pairings are analyzed by computation of relative gain array matrices and scaled condition numbers, which indicate the best pairing choice and the potential of each input-output set. Further, it is possible to minimize the effect of cross couplings and improve stability with the right pairing of input and output. Simulation of selected candidate input-output pairings demonstrate that decentralized control can provide stable operation of the heat pump.

Evaporator superheat control With one temperature sensor using qualitative system knowledge

This paper proposes a novel method for superheat control using only a single temperature sensor at the outlet of the evaporator, while eliminating the need for a pressure sensor. An inner loop controls the outlet temperature and an outer control loop provides a reference set point, which is based on estimation of the evaporation pressure and suitable reference logic. The pressure is approximated as being linear and proportional to the opening degree of the expansion valve. This gain and the reference logic is based on calculation of the variance in the outlet temperature, which have shown to increase at low superheat. The parameters in the proposed controller structure can automatically be chosen based on two open loop tests. Results from tests on two different refrigeration systems indicate that the proposed controller can control the evaporator superheat to a low level giving close to optimal filling of the evaporator, with only one temperature sensor. No a priori model knowledge was used and it is anticipated that the method is applicable on a wide variety of refrigeration systems.

A learning based precool algorithm for utilization of foodstuff as thermal energy storage

Maintaining foodstuff within predefined temperature thresholds is important due to legislative requirements and to sustain high foodstuff quality. This is achieved using a refrigeration system. However, these systems might not be dimensioned for hot summer days or possible component performance degradation. A learning based algorithm is proposed in this paper, which precools the foodstuff in an anticipatory manner based on the saturation level in the system on recent days. The method is evaluated using a simulation model of a supermarket refrigeration system and simulations show that thermal energy can be stored in foodstuff to cope with saturation in refrigeration equipment. Additional hardware or a system model is not required, making it easy to implement the method in existing systems.

Utilization of excitation signal harmonics for control of nonlinear systems

Many model based control methods exist in the literature. Producing a sufficient system model can be cumbersome and a new non-model based method for control of nonlinear systems with input/output maps exhibiting sigmoid function properties is therefore proposed. The method utilizes an excitation signal together with Fourier analysis to generate a feedback signal and simulations have shown that different system gains and time constants does not change the global equilibrium/operating point. An evaporator in a refrigeration system was used as example in the simulations, however, it is anticipated that the method is applicable in a wide variety of systems satisfying the sigmoid function properties.

Inexpensive CubeSat attitude estimation using COTS components and Unscented Kalman Filtering

This paper describes a quaternion implementation of an Unscented Kalman Filter for attitude estimation on CubeSats using measurements of a sun vector, a magnetic field vector and angular velocity. Using unit quaternions provides a singularity free attitude parameterization. However, the unity constraint requires a redesign of the Unscented Kalman Filter. Therefore, a quaternion error state is introduced. Emphasis has been put in making the implementation accessible to other CubeSat by using realistic models of COTS components used for attitude sensing and simulations have shown that the extra computational cost of estimating bias in measurements is worthwhile. The simulations where performed in a simulation environment for the CubeSat AAUSAT3, where robustness has been an important factor during tuning of the attitude estimators. The results indicate that it is possible to achieve acceptable CubeSat attitude estimation, even during eclipse, on a limited budget and using COTS components, without the need of expensive high precision sensor setups.