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[EN] The aim of this study is to estimate the energy savings potential that can be achieved using inflatable dock shelters versus simple curtain dock shelters for loading/unloading activities in logistics warehouses. The article describes how these savings have been analysed and quantified in a big logistics centre of a Spanish dealer. It takes into account different refrigeration applications (i.e. for different warehouse dock temperatures), exterior conditions and daily loading/unloading schedules and their duration. We have used mean typical years for the different Spanish climatic zones, although the procedure can be easily extended to any climate. As expected, the greatest energy savings are achieved in the warmest climates with the coldest storage temperatures and for the nocturnal operations. The savings quantification allowed us to convince the owner to replace the conventional dock shelters. Finally, the implementation of the procedure in a software-tool is helping other dealers to carry out a selfevaluation of the savings potential of their facilities.

[EN] The large amount of recoverable heat from CO2 refrigeration systems has led UK food retailers to examine the prospect of using refrigeration integrated heating and cooling systems to provide both the space heating and cooling to food cabinets in supermarkets. This study assesses the performance of a refrigeration integrated heating and cooling system installation with thermal storage in a UK supermarket. This is achieved by developing a thermal storage model and integrating it into a pre-existing CO2 booster refrigeration model. Five scenarios involving different configurations and operation strategies are assessed to understand the techo-economic implications. The results indicate that the integrated heating and cooling system with thermal storage has the potential to reduce energy consumption by 17¿18% and GHG emissions by 12¿13% compared to conventional systems using a gas boiler for space heating. These reductions are achieved despite a marginal increase of 2¿3% in annual operating costs. The maximum amount of heat that can be stored and utilised is constrained by the refrigeration system compressor capacity. These findings suggest that refrigeration integrated heating and cooling systems with thermal storage are a viable heating and cooling strategy that can significantly reduce the environmental footprint of supermarket space heating provision and under the adequate circumstances can forsake the use of conventional fossil-fuel (natural gas) boiler systems in food-retail buildings.

[EN] This paper compares and quantifies the energy, environmental and economic benefits of various control strategies recovering heat from a CO2 booster system in a supermarket for space heating with the purpose of understanding its potential for displacing natural gas fuelled boilers. A theoretical steady-state model that simulates the behaviour of the CO2 system is developed and validated against field measurements obtained from an existing refrigeration system in a food-retail building located in the United Kingdom. Five heat recovery strategies are analysed by modifying the mass flow and pressure level in the condenser. The model shows that a reduction of 48% in natural-gas consumption is feasible by the installation of a de-superheater and without applying any advanced operating strategy. However, the CO2 system can fully supply the entire space-heating requirements by adopting alternative control strategies, albeit by penalising the coefficient of performance (COP) of the compressor. Results show that the best energy strategy can reduce total consumption by 32%, while the best economic strategy can reduce costs by 6%. Findings from this work suggest that heat recovery systems can bring substantial benefits to improve the overall efficiency of energy-intensive buildings; nevertheless trade-offs need to be carefully considered and analysed on a site by site basis before embarking on such initiatives.

[EN] The paper analyses how to compute the steady-state flow distribution through a given network by using the Minimum Entropy Production (MinEP) principle. For isothermal and incompressible flows, this is equivalent to the minimal dissipation of energy. The conditions that make this method equivalent to the conventional one are studied. There must exist a power-law for the energy dissipation (entropy generation) where the exponent must be the same for the whole network. To our knowledge, Niven was the first to get to this result. However he applied MinEP only to parallel pipes and unfortunately discarded it as a general method. The paper shows why it cannot be discarded yet. We discuss the role of the chosen exponent m and its link to the underlying physical phenomena. Moreover it is shown that there is a ¿hidden¿ fixed point value problem that must be studied further. The method introduced in this paper is developed specially for tree-shaped duct-networks which are frequently encountered in HVAC (Heating Ventilation and Air Conditioning) systems. The paper explains briefly what triggered this research; specifically, difficulties related with branched junctions, mainly in return-type networks.

The IEA agency in its annex 41 deals with the modelling of heat, air and moisture (HAM) transfer in whole buildings. In its words, coupling among these phenomena is crucial for future energy optimization of buildings. Computational fluid dynamics (CFD) is difficult and costly to use for everyday practice. Instead, popular building energy simulation software tools (BES) use simplified models popularly known as multi zone models. Historically the thermal methods were developed separately from the multi-zone airflows and with different purposes. In well-known multi-zone flow software-tools like CONTAM [1] or COMIS [2], the airflow is decoupled from the thermal problem. BES try to gather those separated efforts and couple both heat and the airflow phenomena using different strategies [3]. All this separated development and the increase in computation capacity, has led to a point where improvements are needed [3]. After a critical review of recent hygrothermal zone models, we present a proposal of a simplified zone model for the air-water mixture dynamics which can be used in BES. The resulting dynamical state variables are the zone dry temperature, absolute humidity, total pressure and dry air mass content. This means that, in contrast with other literature models, we employ three intensive and one extensive variable respectively. Moreover, the reference height level inside the zone cannot be chosen arbitrarily. The dynamic is obtained by using zone energy balance, balances of dry air and water mass content and the Equation of State for the wet air mixture. The zone can store dry air and water, therefore their density can change with time inside the volume. Inside the zone a hydrostatic pressure field is assumed with no kinetic energy storage. The resulting model is a made up of three non-linear ordinary differential equations and one algebraic equation. The model is automatically capable of dealing with wet air saturation conditions by using a special control signal al

[EN] A popular method to obtain the 1-D conduction heat transfer of multi-layered plane geometries is attrib- uted to Stephenson and Mitalas (1971) and Mitalas (1968). It is applied in different forms known as; CTF (Conduction Transfer Functions) or RT (Response factors). Roughly his idea consists in sampling the tem- perature at each side of a wall at a certain fixed time step (usually one hour). Between sampling points, due to the lack of information, a linear profile for the evolution of those temperatures is imposed. A tri- angle shaping function is used to get such a piecewise linear profile. The method although is powerful, has passed through the years without questioning. The paper proposes an improvement by extending the idea of a shaping function. Instead of assuming a linear evolution, a specially designed second order (parabolic) evolution is enforced. Now, two parameters must be determined to define the temperature shape between sampling points at both sides of the wall; on one side, the new values of the temperatures and on the other, their acceleration within the time step. Contrary to Mitalas, now two equations are needed at each side to determine these new shapes; the heat power flux balance at both surfaces at the new sampling point (like in Mitalas method) plus the thermal energy balance along the period of time between sampling points. This turns the scheme into an energy conservative one. Finally it is shown that the accuracy obtained by the parabolic case at one hour of sampling rate is similar to the linear case with a sampling rate of five minutes.

[EN] In a previous paper [1] it was shown a proposal for a discrete event simulation (DEVS) model of a thermal zone. A quantized state integrator of order one (QSS) was used to integrate the evolution of the room air temperature. In order to increase the computational speed, this paper presents how the multi-layered wall, 1D conduction heat flow dynamics could be modified to allow its use with a second order QSS2 quantized integrator. The almost forgotten successive transition state method (see U. Yoshimi et al. [2]) is modified with changes in the hold function. Finally to validate the results, a test room has been employed to compare the results with EnergyPlus v7.1. Three cases have been studied: fixed T zone , floating T zone and floating T zone plus a random convective heat gain.

The paper presents a new proposal of a discrete event simulation (DEVS) model for heat transfer in an enclosure. To our knowledge this is the first proposal in this direction, therefore the model is not comprehensive (has no windows, no ground coupling, etcetera). It is a first step of DEVS into the heat transfer in buildings. The model presented is just one possible implementation for a single thermal zone, although it has been designed to be used in multi-zone models. Common methods like CTF (conduction transfer functions) or RTF (response transfer functions) cannot be used. Instead it employs the successive transition state formulation for the 1D conduction heat transfer through multi-layered walls. An example test room has been calculated and the results compared with software that uses well known methods (CTF, RTF). Finally the DEVS model has been tested with a random convective internal-load signal.

This article presents a complete mathematical model which improves the description of the thermal response of a ventilated façade. The model has been validated experimentally for a wide variety of weather conditions; hot and cold days, cloudy and sunny and with clear or dark coloured external board with very good matching. In the validation process the improvement to achieve the matching between the model and the experiences takes into account the influence of the wind direction on the velocity of the air inside the ventilated space.

Due to the increasing energy consumption of air conditioning in buildings and the need to decrease the fossil CO2 emissions to the environment, the interest of using renewable energy sources shows up stronger than ever. We present a general study whose aim is to propose a method to evaluate an upper bound in the potential of solar cooling by using some simplified models. As an example it has been applied to the very diverse climates of Spain. In the paper it has been assumed a direct solar coupling between the solar collector field and a single effect absorption cooling machine, without any intermediate solar storage tank. An equation is obtained that shows the dependence of the generator/solar-collectors equilibrium temperature on basic design parameters of the system (absorption machine-solar collectors). The paper analyzes the effect of these on the total amount of cooling produced along a typical mean year and the peak cooling power. The paper also includes a discussion on how to estimate the values and what is their physical meaning of the parameters which define the behavior of real absorption machines. Finally tables are included for the 12 climates of Spain that can be used as an example of how to make a quick pre-sizing of such direct coupled system. The classification of the Spanish climates is based on general data (average monthly total horizontal solar radiation, average monthly dry temperature, etc.) and the results could be generalized for climates with the same severity. Moreover if hourly weather data is available for any place (like tmy2, bin, epw, etc. files), the procedure can be applied without further changes.