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A general procedure for the optimal integration of buildings and their energy plants

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A general procedure for the optimal integration of buildings and their energy plants
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  Claudia Toro, claudia.toro@uniroma1.it A general procedure for the optimal integration of buildings and their energy plants  E.Cheremnykh 1  , M. Cianfrini  2  , C. Toro  2,   1  University of Roma “Tor Vergata”, Italy 2  University of Roma “La Sapienza”, Italy Abstract:  The sustainable energy conversion in space conditioning systems has become an urgent issue on the energy agendas of most developed countries. This sector is one of the largest energy consumers and also, unfortunately, one of the least efficient from the point of view of  primary-to-end-use matching. The most recent industrial solutions, frequently based on renewable sources, have the potential to remedy the situation as long as a comprehensive analysis of the energy system consisting of the building and of its thermal/mechanical plants is carried out. The  present study presents a general systemic procedure for the optimal integration of buildings and their energy plants (heating/cooling element + primary energy supply system) that allows for the identification of the thermodynamically most convenient configurations. The method combines a thermal building dissipation modeling with a numerical simulation of the thermal consumption and an exergy efficiency calculation. Keywords :  space conditioning, green buildings, exergy efficiency, final energy use, source/end-use matching 1.Introduction A series of recent studies ([9],[12],[11] and [1])focused on building thermal design have proposed and tested concepts of advanced modeling and analysis tools that include Second Law considerations. These studies revolutionary integrate the thermal characteristics and demands of a building or any living space, with energetic and environment performances of air conditioning system. In these studies the approaches to exergy analysis and performance indicators differentiate from each other, and, therefore, lead to diversifies conclusions. The scope of present study is to develop and test a general systemic procedure for the optimal integration of  buildings and their energy plants. The thermal building dissipation modeling enable to precise the actual thermal demand of a living space, afterwards, by proper simulations tools, to identify the most exergetically suitable energy plant. The “optimal” configuration for an air conditioning system depends on several factors: the thermal characteristics of the building to be conditioned, the type of internal heating element (convector, ceiling or floor    radiant panel in this study, since the example of application is limited to the winter mode of operation) and the type of the primary energy conversion system (solar collector, PV and PVT, heat pump,etc.). The first step of the method proposed here is the modelling and thermo-fluidodynamic simulation,  performed via a commercial CFD code (Fluent®), of the effects of the above mentioned internal heaters integrated into a simplified building geometry. The temperature maps obtained via these CFD simulations are then used to compute, for each type of heating element, the actual thermal power required to meet the environmental comfort standards. The second step is the simulation of the global energy plant needed to provide the calculated heat demand,  performed by means of a process simulator (CAMEL-Pro™). This global simulation enables the designer to compare the performance of all feasible different combinations of internal and external systems in order to identify the most exergetically convenient pairings for a better primary source/end-use matching. 2. Numerical tools To perform a detailed analysis of the thermal characteristics of the building, it is necessary to know the  building temperature map, so that a more accurate value of the heat demand can be obtained based on the correct calculation of the convective and radiative heat transfer on the inner surfaces. Notice that such thermal maps depend substantially on the type of heating/cooling device we choose: therefore, the calculations must be repeated for each of the possible/feasible configurations. All thermo-fluid dynamic simulations presented in this work have been performed via a commercial CFD code (Fluent® [3]). The pre-processing software Gambit, embedded with Fluent, has been used to create geometry and generate grid. The systemic approach, consisting on geometry modelling, mesh creation and automatic acquisition by importing in Fluent, could be operated for all type of heating elements. Since the internal flows are likely to be turbulent, the standard k    ε  model of Hanjalic/Launder/Spalding has  been employed [4]. This well established model is based on model transport equations for the turbulence kinetic energy (k) and its dissipation rate ( ε ). The model transport equation for k is derived by modelling the exact equation, while the transport equation for ε  is obtained using flow similarity considerations and proper energy considerations, and bears little resemblance to its exact counterpart. In the derivation of the k    ε  model, the assumption is that the flow is fully turbulent, and the effects of molecular viscosity are negligible. The model is therefore valid only for fully turbulent flows. The pressure-velocity coupling is handled through the SIMPLE-C algorithm described by Van Doormaal and Raithby [13] , a refined variant of the SIMPLE algorithm previously developed by Patankar and Spalding [7]. The advection fluxes are evaluated by the QUICK discretization scheme proposed by Leonard [5]. The computational spatial domain is filled with a non-uniform grid, with a higher concentration of grid lines near the  boundary walls and high-gradient areas, and a lower uniform spacing throughout the remainder interior of the domain. After convergence of the velocity and temperature fields, the amount of thermal power transferred from the heated portion of the floor or ceiling to the enclosed space is calculated. As stated above, the results of the CFD simulations provide the thermal maps of the interior, that are then used to: a) verify that the comfort zone fits well with the usual occupancy areas;  b) calculate the actual thermal load of the building. To perform the process simulation of each type of building conditioning system and calculate the power consumption and exergy efficiency of each configuration external and internal sub-units models have been implemented and integrated in the CAMEL-Pro™ [15] simulation software. CAMEL-Pro™ is written in C#, is  based on an object-oriented approach, and is equipped with a user-friendly graphical interface that allows for the simulation and analysis of several energy conversion processes. The system is represented as a network of components connected by material and energy streams; each component is characterized by its own set of equations describing the thermodynamic changes imposed on the streams. 3 Exergy Concepts  Exergy can be defined as the maximum amount of work which can be obtained from a system or a flow of matter when it is brought reversibly to equilibrium with the reference environment. For each component of a plant, the outlet exergy is always less than the inlet exergy because of irreversible  processes. When calculating the exergy of a process component, the difference between the exergy losses and exergy destruction are recorded. Exergy losses include the exergy flowing to the surroundings, whereas exergy destruction indicates the loss of exergy within the system boundary due to irreversibility. The exergy of a stream of matter can be divided into different “component exergies”. In the absence of nuclear, magnetic, electrical and surface tension effects, exergy is calculated as the sum of : KPPhCh  ExExExExEx      (1) where Ex K  , Ex P  , Ex th  and Ex ch  are the kinetic, potential, physical and chemical exergy respectively. The changes in the kinetic and gravitational potential energies are neglected in the present study. Physical exergy is defined as the maximum amount of work which can be obtained when a stream of matter is brought from its initial state to the environmental state while only exchanging heat with the thermal reservoir of the environment, whereas chemical exergy is defined as the maximum amount of work which can be obtained when the stream of matter is brought from the environment state to the total dead (unrestricted) state as a result of heat transfer and exchange of substances only with the environment.     000 Ph  ExhhTss       (2) 00, ··ln chiiichi  ExRTxxxex       (3) where x i  is the mole fraction of the species i in the flow and ex ch,i0 is the molar chemical exergy of the i th  species.  To perform an exergy analysis of the air conditioning plants studied in this work, we need to calculate first the mass- and energy flows of each process. The process simulator (CAMEL-Pro™) calculates the exergy of each (material and immaterial) stream and obtains the values of the exergy destruction, Ex λ  and of the exergy efficiency, η ex , of each component. external   plant   internal   plantT,   cold w T,   hot w .;   P exergy   source       e      x T   room N   personP light T e ;   Solar   heat,   kWVentilationHeat   Losses   to   the   surroundings       e      x      e      x   Figure 1 Exergy flows within the “building” system 4 Description of the case studies 4.1 Layout To show the potential of the proposed methodology four case studies will be presented here. All deal solely with the “winter mode” operation, in which the building is heated against a pre-specified lower ambient temperature. The choice of heating devices were guided by market considerations. As external plants, a heat pump , a solar collector and an hybrid (PVT) solar collector are compared. For the internal heating devices, ceiling and floor radiant panel and fan coil were considered. The seven cases are:    CASE 1) Solar collector coupled with radiative floor panel; CASE 2) Heat pump coupled with radiative floor panel; CASE 3) PVT coupled with radiative floor panel; CASE 4) Solar collector coupled with radiative ceiling panel; CASE 5) Heat pump coupled with radiative ceiling panel; CASE 6) PVT coupled with radiative ceiling panel; CASE 7) Heat pump coupled wuth fan coil. The system layouts are shown in Figure 2 ,3, 4 and 5. SOLARCOLLECTOR TANK RADIANT HEATER PIPES PUMP 1 PUMP 2 POWER POWER SOLAR IRRADIANCE HEAT  Figure 2 Solar collector coupled with Radiant Heater (floor Case 1 and ceiling Case 4) Figure 3 Heat Pump coupled with Radiant Heater ( floor Case 2 and ceiling Case 5) PIPES RADIANT HEATER PUMP 2 PUMP 1 POWER POWER POWER HEAT TANK AIR Flow    Figure 4 PVT coupled with Radiant Heater ( floor Case 3 and ceiling Case 6) Figure 5 Heat Pump coupled with Fan Coil ( Case 7) In Cases 1 and 4 the external energy inputs considered are the electrical power for water pumps (stream 9 and 3 in Figure 2) and the solar irradiation on the collector (stream 5); the output stream is the heating load of the  building (stream 14). In Cases 3 and 6 the external energy inputs are the electrical power for water pumps (stream 9 and 3 in Figure4) and the solar irradiation on the PVT (stream 5); the output stream are the heating load of the building (stream 13) and the electric current generated by the PVT (stream 21). In Case 2and 5 the external energy inputs are the electrical power for water pumps (streams 21 and 17 in Figure 3) and heat pump (stream 3) and the heat pump inlet air ( stream 8); the output streams are the heating load of the building (stream 15) and the air outgoing from the heat pump (stream 9). In case 7 the exergy inputs are the electrical power for water pumps (streams 15 and 19 in Figure 5) and heat pump (stream 3), the heat  pump inlet air ( stream 8) and the fan coil inlet air (stream 22); the output stream are the fan coil hot outlet air (stream 23) and the heat pump outlet air (stream 8). To simulate their operation the described seven plants have been modelled with the CAMEL-Pro™ process simulator. Details about thermodynamic models implemented for internal and external components within the simulator are reported in [1]and in [10] . PVTTANKPIPES   RADIANT HEATER PUMP 1 PUMP 2 Power Power HEAT SOLAR IRRADIANCE Electric Power PIPESPUMP 1 PUMP 2 FAN COIL Power Power Power External Air AIR TANKPower
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