Exergy assessment of an optimized capillary tube-based transcritical CO₂heat pump system

Abstract

SUMMARYA capillary tube-based CO 2 heat pump is unique because of the transcritical nature of the system. The transcritical cycle has two independent parameters, pressure and temperature, unlike the subcritical cycle. A comparative study for various operating conditions, based on system COP and exergetic efficiency, of a capillary tube and a controllable expansion valve-based transcritical carbon dioxide heat pump systems for simultaneous heating and cooling at 73 and 41C, respectively, is presented here. Two optimized capillary tubes having diameter of 1.5 and 1.6 mm are compared with an equivalent controllable throttle valve. Heat transfer and fluid flow effects are included in the gas cooler and evaporator model and capillary tube employs the homogeneous flow model to simulate two-phase flow. Subcritical and supercritical thermodynamic and transport properties of CO 2 are calculated employing a precision in-house property code.Optimization of effective distribution of total heat exchanger area ratio between gas cooler and evaporator is investigated. The exergetic efficiency is better in case of the capillary tube than that of a controllable throttle valve-based system. Capillary tube-based system is shown to be quite flexible regarding changes in ambient temperature, almost behaving to offer an optimal pressure control just like the controllable expansion valve yielding both, maximum system COP and maximum exergetic efficiency. Relatively at a smaller diameter, the capillary tube exhibits better exergetic efficiency. Capillary tube length is the critical parameter that influences system optimum conditions. The exergy flow diagram exhibits that compressor, gas cooler and capillary tube contribute a larger share, in that order, to system irreversibility. It is fairly established in this study that a capillary tube can be a good engineering option for small capacity systems in lieu of an expansion valve, which has been thought of as the only possible solution to attain the pressure optimization, an important feature of all transcritical CO 2 systems.

Performance study on hybrid solar-assisted CO2 heat pump system based on the energy balance of net zero energy apartment

“…Moreover, research data regarding the exergy efficiency from Refs. [31][32][33][34] are presented for comparison. The influence of certain environment parameters on the cooling performance, e.g., the effect of ambient temperature or tank driven temperature on the COP, is also discussed in the first part.…”

“…The exergy efficiency (EE) analysis has already been conducted in the previous evaluations of CO 2 refrigeration, air-conditioning, and HP systems [31][32][33][34]. In order to compare the EE of the proposed hybrid CO 2 HP cycle with that of a conventional CO 2 HP cycle, calculated results from the EES model and results from other references [31][32][33][34] are listed in Table 5.…”

“…In order to compare the EE of the proposed hybrid CO 2 HP cycle with that of a conventional CO 2 HP cycle, calculated results from the EES model and results from other references [31][32][33][34] are listed in Table 5. In summary, there are six groups of data for EE comparison, including the conventional CO 2 HP (Case C1, C2, and C3), the hybrid CO 2 HP (H1), and the conventional CO 2 HP with an internal heat exchanger (I1 and I2).…”

Turbulent heat transfer characteristics of supercritical n ‐decane in a vertical tube under various operating pressures

“…More attention is primarily focused on carbon dioxide, water, refrigerants, and hydrocarbon fuels, particularly for water and carbon dioxide in the view of the heat transfer. In addition, there are also studies on thermodynamic analysis . However, the effect of hydrocarbon fuels at various conditions on heat transfer characteristics gets more attention with the development of scramjet engines .…”

Numerical analysis of transcritical carbon dioxide compression cycle: a case study

“…One excellent example is micro-channel heat exchangers which reduce the cost of system. Using the technology, the dimensions of carbon dioxide refrigeration system has become several times smaller than conventional systems with the same capacity and efficiency [1]. Carbon dioxide refrigerant is currently used in two types of refrigeration cycle.…”