One of the challenges of today is to achieve large amounts of electrical energy storage. This can be reached with the help of pumped heat energy storage (PHES) technology, where electricity is used to supply a heat pump system connected to two large thermal tanks. The system consists of four stages: compression, high temperature heat exchange (delivering in pump mode or absorbing in engine mode), expansion, and low temperature heat exchange (absorbing in pump mode or delivering in engine mode). From a technological point of view, the major issues to solve are the compressor and expander. Numerical simulations can be an important tool to quantify its theoretical potential. Numerous studies that simulate PHES systems use constant isentropic efficiencies to represent the compression and expansion stages. In this work a zero-dimensional model for a reciprocating compressor or expander is presented. Its objective is to represent the real behaviour of these stages and determine the main variables of the system. Therefore, a computer routine has been developed, which solves the system of differential energy equations and the mass balance as functions of the rotational speed. The model was validated using a domestic compressor and an industrial expander. To be able to reproduce the compressor case, a dynamic valve model that opens and closes according to the pressure of the system was implemented. In both cases a good agreement with experimental data from the literature was obtained. With the present model, realistic values were obtained for the parameters that characterize the global irreversibility of the system, as well as their behaviour in relation to the crankshaft angle (or the elapsed time), an issue not usually well established in theoretical models. The dynamic model can be integrated in a PHES system simulation in order to represent its transient behaviour in a more realistic fashion.