Economically optimal dimensioning of a counterflow and crossflow heat exchanger for free cooling

During the cold seasons, it is possible to use ambient air to cool down the coolants in air-conditioning or other refrigeration applications. Applying this free cooling reduces the energy costs of a refrigeration machine. This paper presents a general method for the economically optimal dimensioning of a free cooling system in which a heat exchanger of the counterflow or crossflow type is applied to the heat transfer between the ambient air and the coolant. The following costs are taken into account in deriving the method: 1) the investment and annual maintenance costs of the free cooling system, 2) the energy costs in order for the pumps and fans to overcome the pressure losses in the heat exchanger for the free cooling, and 3) the avoided energy costs of the refrigeration machine. The optimization is based on maximizing the net present value of the economical gain obtained from using free cooling. As a result of the optimization using the presented method, economically optimal values are obtained, for instance, for the recuperation ratio and dimensionless conductance of the heat exchanger, as well as for the design temperatures for the dimensioning of the free cooling system. The method proposed here for the economically optimal dimensioning of the free cooling system improves the quality of the design and increases the profitability of the investments made in it while saving finite resources. One advantage of the proposed method is that a customer-oriented investment strategy and investment criteria, such as the interest rate, operating time and local price of electricity, can easily be taken into account in the dimensioning of the free cooling system.