Download
1 / 19
190 likes | 446 Views
Energy Saving Control-Strategies for Reefer Containers Kresten Kjær Sørensen, Lodam electronics. Reefer Power Consumption. There was 11.4 million TEU of reefer containers by mid-2008 and this is predicted to be 69% larger by 2013 [Marine link, 2009].
E N D
Energy Saving Control-Strategies for Reefer Containers Kresten Kjær Sørensen, Lodam electronics
Reefer Power Consumption • There was 11.4 million TEU of reefer containers by mid-2008 and this is predicted to be 69% larger by 2013 [Marine link, 2009]. • With an average power consumption of 3.6kW/TEU the combined consumption is about 41GW. • The size of the fleet is about 4500 ships, giving an average reefer power consumption of 3MW/ship, with 3000 running hours per container per year. • Compared to the power required to propel the ship this is not much but because of the increased focus on the environmental impact of shipping and fuel costs it has become more interesting to optimize efficiency.
Control Modes – The Classics • Two modes: Chill and Freeze • Chill mode is used from -5°C and up • Sensible foodstuffs such as fruit and vegetables • The temperature of the supply air must be controlled • Continuous operation • Freeze mode is used below -5°C • Frozen foodstuffs, less sensible to small temperature variations • The temperature of the return air is controlled • Continuous operation or on/off operation.
Regulating the Cooling Capacity • Because the reefer is required to work over a large temperature range on the hot and the cold side it is necessary to regulate the cooling capacity. • The capacity can be regulated by using: • Compressor with VLT • Suction line modulation • Hotgas valve • On/Off control
Reefer Box Energy Balance • There are four factors that contribute to heating the box • Heat from the ambient air and direct sunlight • Heat from the evaporator fans • Heat from cargo decay/ripening • Heat from defrosting the evaporator • All of these must be removed by the refrigeration plant so it makes sense to try to reduce them. • From a control point of view we can do something about the contributions from the fan and defrosting.
Optimization of Defrosting • If defrosting is done too often energy is wasted on thawing the evaporator with little gain in evaporator efficiency because of insignificant ice buildup. • If defrosting is not done often enough energy is wasted due to an inefficient evaporator because of excessive ice buildup. • Defrosting at the right moment requires knowledge of the amount of ice on the evaporator • From measurements • From a model • Or a combination
Optimization of Fans • The fans circulate the air in the container and if they are not running hot pockets of air may start to appear locally. • The fans are usually not controlled by a VLT but may be turned Off or set to high or low speed. • When the ambient temperature is low the heat contribution from the fans become significant and control strategies that allows the fan to be turned off have shown great reductions in power consumption. • VLT on the fans to compensate for 60Hz supply.
Refrigeration Plant Optimization • The usual good practices for refrigeration control still apply • Low superheat • Optimal condensation pressure • Correct refrigerant charge • Optimal use of the economizer • Quickly to set-points after startup
QUEST 1/2 • Developed by University of Wageningen, The Netherlands • A method of energy optimization for reefers by allowing a larger variance in the supply air temperature and running the compressor in on/off mode • The reefer is designed to both freeze and cool and therefore it is less efficient in part load as when cooling fruit and vegetables • Builds on the hypothesis that the produce in the container is insensible to short bursts of cold air well below the set-point. • The temperature fluctuations is dampened by the packaging and the slow responses of the produce metabolic rates.
QUEST 2/2 • The temperature limits are dependent on the produce in the container • The settings has been designed such that they are dependent on set-point and hence easy to operate • The largest gains are for refrigeration systems without a VLT on the compressor because they run very inefficiently in chill mode. • For a forty foot container loaded with mandarins a reduction in power consumption from 5.6 to 2.7kW has been observed. • The gains come from increased efficiency of the refrigeration plant when it is running, and the fan speed reduction.
Energy Saving in Freeze Mode • In freeze mode the plant is either running continuously or in On/Off mode • In on/off mode it is important that the plant runs efficiently during the entire On period • The COP is dependent on the temperature difference between the cold and hot side. • This means that daily variations in ambient temperature can be exploited, if it is allowed to cool extra at night and “store” some cooling in the produce by lowering its temperature. • If greater temperature variances of the produce is allowed longer off periods is also possible, resulting in a larger COP due to an increased ratio of cooling power to fan power. (Assuming that the fans are turned off or at low speed when not cooling)
Using Models for Optimization • A good system model can be used as a basis for optimal controller design or to make simulated controller tests. • It is hard to completely replace field tests but initial controller development can be done using a model reducing the time one has to wait for the results • Models can be used as observers to optimize control performance in many different ways: • Predictive Control that optimize over several days • For prediction of ice buildup • For nonlinear feed-forwards • For prediction of air temperature while the evaporator fan is turned off • For prediction of cargo temperature and produce decay • For prediction of ambient temperature
End Notes • There are room for improvement but for the well performing reefers the gap is getting small • The efficiency gains still left for well performing reefers require advanced control strategies, and maybe also relaxation of rules. • QUEST is a step in the right direction with respect to relaxation of rules, and research into produce quality vs. temperature variance is both relevant and important • Rules can not be relaxed without thorough and well documented tests of how much temperature can be varied without spoiling the produce.
