Heat pump and refrigeration cycle

Thermodynamic heat pump cycles or refrigeration cycles are the conceptual and mathematical models for heat pump, air conditioning and refrigeration systems. A heat pump is a mechanical system that allows for the transmission of heat from one location (the "source") at a lower temperature to another location (the "sink" or "heat sink") at a higher temperature. Thus a heat pump may be thought of as a "heater" if the objective is to warm the heat sink (as when warming the inside of a home on a cold day), or a "refrigerator" or “cooler” if the objective is to cool the heat source (as in the normal operation of a freezer). In either case, the operating principles are similar. Heat is moved from a cold place to a warm place. Thermodynamic cycles According to the second law of thermodynamics, heat cannot spontaneously flow from a colder location to a hotter area; work is required to achieve this. An air conditioner requires work to cool a living space, moving heat from the inter

Data-based identification of a parametric heat pump model

Franca Schmid

At the LENI a heat pump model has been developed, which aims to simulate a single state compression heat pump in both steady state and transient regime. For gaining the experimental data and for the simulation the same set of input data is used. The goal is to approximate the output values of the model ym to the ones measured in the experiments. Therefore a certain number of parameters need to be defined in order of obtaining the highest accuracy possible. At the beginning these different parameters γ need to be found and a criterion needs to be appointed which can be used to judge the accuracy of the model. The first approach to the identification is to only work with the steady states. This includes to select steady values out of a data sheet and to change the model so that only steady states are computed. As soon as the model is successfully running for the steady states an optimizer needs to be chosen to finally output the best values of parameters. Subsequently the improvement of the output vector ym needs to be checked. As during the examination some computational problems have been revealed, which interfered with the optimization, various influences on the heat pump model were investigated. It was tried to find the reason for numerical problems and to eliminate those in order to finally be able to optimise the parameters. To identify the model in the transient regime the model needs to be linearised at a typical operation condition. The new chosen criterion must also take into account the transient regimes and the optimality of the solution has to be proved. The numerical problems which exist for the steady state also influence the transient regime. As the emphasis was therefore put on finding the origin of the instabilities, the identification of the transient regime was only briefly worked on.

2010

Experimental investigation of electrical domestic heat pumps equipped with a twin-stage oil-free radial compressor

Jean-Baptiste Carré

The world energy consumption has dramatically increased since the end of the World War II. The main share of this energy comes from fossil fuel combustion. The worldwide domestic sector consumes about 30% of the global energy supply and the biggest share of that is dedicated to space and water heating. In this context where sustainable energy consumption is required to limit our impact on the environment and the climate, energy efficiency is a key factor to successfully transition to energy sobriety. Electrically-driven heat pumps are known to be a key technology to increase our energy efficiency. More efficient, more compact, more silent heat pumps, built with less raw material, and using lower refrigerant charges are needed. In electrically-driven heat pumps, the compression process is responsible for most of the energy losses. However, for decades now, the heat pump performance has been stagnating, mainly because of the compression process efficiency which has not been increasing significantly. Consequently, the improvement of the compression process is indeed needed. A new single-stage compression unit design has been developed, built, and tested in a previous thesis work. This current thesis work uses a twin-stage successor of the initial single-stage compression unit and tests it into two oil-free domestic heat pump prototypes. This work aims at studying the integration of the radial compression units in domestic heat pumps and at demonstrating their feasibility and potential. The tested prototypes are a twin-stage Air/Water domestic heat pump and a twin-stage Brine/Water domestic heat pump. Both of them are equipped with a twin-stage oil-free radial compression unit rotating on gas bearings. By which, the maximum rotor speed of the compression units is 180 krpm. Six stable operating points have been documented with the Air/Water heat pump prototype. More notably so, the operating point A-7/W35 has been reached and demonstrates a coefficient of performance of 2.36 for a heating power of 10.7 kW. The Brine/Water heat pump prototype has been used to perform partial tests of circuit improvements, in order to solve some of the issues observed on the Air/Water prototype. The analysis of the experimental results uses a mass and energy balance modeling approach to improve the level of understanding of the internal and non measurable flows in the heat pump circuits. This model also propagates the uncertainties of the measurements through the equations. New improvements and innovative circuit layouts, notably at the level of the economizer, a key-component of the two-stage heat pump circuit, are offered to integrate further the compression unit in the heat pump circuits, and to make the whole system more efficient, more compact, more silent, less demanding in raw materials and reducing the refrigerant charge needed for the heat pump cycle.

EPFL2015

Screening for Refrigerant Blends in High-Temperature Lift Heat Pumps for Retrofit in Residential Heating

Daniel Favrat, Michele Zehnder

Providing heat pumps for the retrofit of fuel based boilers in residential heating, is a short or medium term challenge and a major opportunity for a substantial reduction of greenhouse gas emissions. While effective solutions for improved high temperature lift, vapor compression heat pumps have been analyzed elsewhere, this paper specifically deals with the refrigerant selection aspects for alternative blends, using a multi-objective optimization algorithm. The objectives COP and specific heat output are calculated for arbitrarily mixed refrigerant blends, using 17 selected pure fluids. Seasonal mean operating conditions (ambient air 0°C, heating water 50°C) have been used to simulate the heat pump cycle. Blends with a saturation curve extending over the range of –20°C (normal boiling point) and 70°C (critical point) are considered. Alternative blends are compared with the commercial mixture R-407C. The refrigerant mixture with the highest COP shows an improvement of 8% on the low capacity end of the Pareto-optimum curve. By maximizing the specific heat rate, a gain of 83% can be achieved. All the dominating solutions are composed of flammable refrigerants.

2005

Experimental investigation of electrical domestic heat pumps equipped with a twin-stage oil-free radial compressor

Jean-Baptiste Carré

EPFL2015