Jørgen Bauck Jensen

Optimal operation of simple refrigeration cycles part i : Degrees of freedom and optimality of sub-cooling

The paper focuses on the operation of simple refrigeration cycles. With equipment given, there are, from a control and operational point of view, five steady-state degrees of freedom; the compressor power, the heat transfer in the condenser, the heat transfer in the evaporator, the choke valve opening and the active charge in the cycle. Different designs for affecting the active charge, including the location of the liquid receiver, are discussed. With a given load (e.g. given cooling duty) the compressor power is set. Furthermore, it is usually optimal to maximize the heat transfer. The two remaining degrees of freedom (choke valve and active charge) may be used to set the degree of super-heating and sub-cooling. It is found that super-heating should be minimized whereas some sub-cooling is optimal. For a simple ammonia cycle, sub-cooling gives savings in compressor power of about 2%. In this paper, refrigeration (cooling) cycles are considered, but the same principles apply to heat pumps.

Optimal operation of simple refrigeration cycles Part II: Selection of controlled variables

The paper focuses on operation of simple refrigeration cycles and considers the selection of controlled variables for two different cycles. One is a conventional sub-critical ammonia refrigeration cycle and the other is a trans-critical CO2 refrigeration cycle. There is no fundamental difference between the two cycles in terms of degrees of freedom and operation. However, in practical operation there are differences. For the ammonia cycle, there are several simple control structures that give self-optimizing control, that is, which achieve in practice close-to-optimal operation with a constant setpoint policy. For the CO2 cycle on the other hand, a combination of measurements is necessary to achieve self-optimizing control.

OPTIMAL OPERATION OF A SIMPLE LNG PROCESS

Abstract Considering the large amount of work that goes into the design of LNG processes there is surprisingly little attention to the subsequent operation. This probably comes from the misconception that optimal design and optimal operation is the same, but this is usually not true. In this paper we are studying optimal operation of a relatively simple LNG process, namely the PRICO process.

Optimal operation of a mixed fluid cascade LNG plant

Studies on the operation of complex vapour compression cycles, like the one used for the production of liquefied natural gas (LNG), are not widely reported in the open literature. This is a bit surprising, considering the large amount of work that has been put into optimizing the design of such processes. It is important that the process is operated close to optimum to fully achieve the maximum performance in practice. These are possibilities for savings, both due to (a) identifying the optimal point of operation, and (b) selecting the controlled variables such that the optimal operation depends weakly on disturbances. In this paper we study the mixed fluid casade (MFC) LNG process developed by The Statoil Linde Technology Alliance . We study the degrees of freedom and how to adjust these to achieve optimal steadystate operation.

Single-cycle mixed-fluid LNG process Part II: Optimal operation

Considering the large amount of work that goes into the design of LNG processes, there is surprisingly little attention to their subsequent operation. This partly comes from the assumption that optimal design and optimal operation are the same, but this is not generally true. In this paper we study the optimal operation of a relatively simple LNG process, namely the PRICO process.

Single-cycle mixed-fluid LNG process Part I: Optimal design

In this paper, we discuss the design optimization of a relatively simple LNG process; the PRICO process. A simple economic objective function is stated and used on eight different cases with varying constraints. Important constraints are discussed and the results are compared with the commercial PRICO process and with other publications.

Control and optimal operation of simple heat pump cycles

Cycles for heating and cooling have traditionally been studied in detail when it comes to thermodynamics and design. However, there are few publications on their optimal operation which is the theme of this paper. One important issue is which variable to control, for example, degree of super-heating, pressure, liquid level or valve opening. Also, unlike open systems, the initial charge to the cycle may have a steady state effect, and it is discussed how different designs are affected by this factor. Numerical results are provided for an ammonia cycle.

On the trade-off between energy consumption and food quality loss in supermarket refrigeration systems

This paper studies the trade-off between energy consumption and food quality loss, at varying ambient conditions, in supermarket refrigeration systems. Compared with the traditional operation with pressure control, a large potential for energy savings without extra loss of food quality is demonstrated. We also show that by utilizing the relatively slow dynamics of the food temperature, compared with the air temperature, we are able to further lower both the energy consumption and the peak value of power requirement. The Pareto optimal curve is found by off-line optimization.