Abstract
The combined heat pump–organic Rankine cycle is a thermal–electrical storage concept which allows the reversible use of components in both operation modes (loading and unloading the storage). This saves in terms of investment costs but also creates challenges during design and operation. A heat exchanger is an expensive component destined to be used for the reversible purposes of a heat pump condenser and an organic Rankine cycle evaporator. In this study, the operation of such an apparatus was evaluated based on an analytical model, experimental data and thermal imaging. This study shows that the model can predict the filling of the apparatus distinguished by liquid, vapour and the two-phase region. The thermal imaging supports the model and gives the location of the regions. Connecting both methods, a valid statement about the current condition of the heat exchanger is possible. Due to very small pinch points, the apparatus is not efficiently used in the investigated modes. Extending the pinch to 2 K can already save up to 46.1% of the heat exchange area. The quality of the heat transfer in the evaporator (q˙ORC = 10.9 kW/m2) is clearly higher than in the condenser (q˙HP = 6.1 kW/m2).
Highlights
The energy transition towards a CO2-neutral future requires a growing amount of renewable energy sources
This study shows that the model can predict the filling of the apparatus distinguished by liquid, vapour and the two-phase region
The following procedure shows an analytical method of determining the filling of the heat exchanger, which operates as an heat pump cycle (HP)-condenser and organic Rankine cycle (ORC)-evaporator
Summary
The energy transition towards a CO2-neutral future requires a growing amount of renewable energy sources. Solar and wind power produce a fluctuating energy supply and cannot replace the fossil energy production without any changes to the distribution grid and organisation. To shift energy from times of surplus production to times of need, energy storage concepts are necessary. In medium- and large-scale applications for midterm storage duration (hours to days), the use of pumped thermal energy storages (PTES) is increasing. These employ Carnot batteries which are presented with different concepts for a huge range of applications [1]. The heat pump cycle (HP) converts low temperature heat into high temperature heat by the use of electrical energy. The organic Rankine cycle (ORC) converts high temperature heat into electrical energy and releases low temperature heat.
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