博物馆空调的节能策略.docx

Contents lists available at ScienceDirectApplied Thermal Engineeringjournal homepage: /locate/apthermeng Applied Thermal Engineering 29 (2009) 676686Energy saving strategies in air-conditioning for museumsFabrizio Ascione, Laura Bellia, Alfonso Capozzoli, Francesco Minichiello *DETEC, University of Naples Federico II, P.le Tecchio 80, 80125 Naples, Italya r t i c l e i n f o Article history:Received 2 February 2007Accepted 24 March 2008Available online 31 March 2008Keywords:Museum Microclimatic control HVAC systemsEnergy saving strategiesa b s t r a c t In the museum environment a strict thermal-hygrometric control is necessary primarily for the correct artwork conservation and then for the visitor thermal comfort. Considering that the air-conditioning sys- tem has to operate constantly, suitable techniques permit to obtain useful energy savings, allowing, how- ever, a good dynamic microclimatic control.In this paper a case study is presented about various strategies used to reduce energy requirements for HVAC systems in an exhibition room of a modern museum. Using the dynamic simulation code DOE 2.2 and typical climatic hourly data sets, the annual energy use for an all-air system has been calculated, as well as the savings obtainable using different techniques, such as dehumidication by adsorption (des- iccant wheel saving equal to 15% with respect to a base conguration), total energy recovery from the relief air (passive desiccant 15%), outdoor airow rate variation (demand control ventilation 45%). Moreover, the correspondence has been analyzed between the energy request and the admitted variation of indoor temperature and relative humidity: changing the admitted indoor RH range from 50 2% to 50 10%, energy savings around 40% have been obtained. As regards the thermal-hygrometric performance, an optimal control of temperature has been guaranteed with all the congurations, while the best performance in RH control has been obtained with the desiccant system.Considering a simple payback analysis, if the artworks preserved in a museum are particularly sensitive to indoor humidity variation, a desiccant system should be properly used; on the contrary, when the indoor humidity control is not strongly needed, the use of a HVAC system with demand control ventila- tion is advisable, because of the lowest payback value. The system with total energy recovery presents intermediate features. 2008 Elsevier Ltd. All rights reserved.1. IntroductionThe conservation of the artworks requires the control of the in- door microclimatic conditions to limit degradation phenomena. Thus, a suitable HVAC system is often necessary for the museum environment, in order to guarantee safety values of indoor ther- mal-hygrometric parameters and air velocity and, over all, to min- imize the changes of these parameters from the design values. The main causes of the dangerous variations of the microclimatic con- ditions in the exhibition rooms are the thermal loads due, respec- tively, to the outdoor air (ventilation load) and to the visitors; the last one depends on the degree of overcrowding and is generally greatly variable 14.On the other hand, considering the continuous working of the air-conditioning system, during all the year and 24 h per day, the adoption of suitable techniques is useful to obtain considerable en- ergy savings 3,5.* Corresponding author. Tel.: +39 081 7682533; fax: +39 081 2390364.E-mail addresses: fabrizio.ascioneunina.it (F. Ascione), belliaunina.it (L. Bellia), alfonso.capozzoliunina.it (A. Capozzoli), minichieunina.it (F. Minichiello).In this paper, a case study relative to the HVAC system for a sim- ulated modern museum is presented. The operating costs of an all- air system have been evaluated, using different energy saving strategies, such as the dehumidication by adsorption, the total energy recovery from the relief air, the demand control ventilation. For this purpose, the dynamic simulation code DOE 2.2 6 has been used, as well as typical climatic hourly data TRY 7. The ef- fects of these energy saving strategies on indoor T and RH control have been also analyzed, as well as the correlation between the po- tential annual energy savings and the admitted variation of indoor T and RH. Finally, a simple payback analysis has been carried out, in order to know whether the energy conservation measures pro- posed are cost effective.2. Microclimatic control in museums2.1. Microclimatic requirementsInteractions between the museum and the outside environ- ment, if not opportunely controlled, may accelerate the processes1359-4311/$ - see front matter 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.03.040NomenclatureBASECAV DCVbase HVAC system used as comparison referenceconstant air volumedemand control ventilation (i.e. outdoor airow rate variation)desiccant wheelenergy recovery ventilator (i.e. total energy recovery, or passive desiccant, or enthalpy wheel)RHSm3TPIair relative humidity (%)standard m3 (m3) temperature (C) performance index (%)DWERVF. Ascione et al. / Applied Thermal Engineering 29 (2009) 676686677of deterioration, often irreversible, of both the building envelope it- self and the artworks within.The principal risks to the cultural heritage come from degrada- tion phenomena, which may mean mechanical mechanisms (changes of size and form of objects), chemical reactions and bio- logical mechanisms (proliferation of micro-organisms) 8.The main agents responsible for the degradation processes of historical buildings and artworks are 3,9: thermal-hygrometric conditions and vertical thermal distribution of air masses; velocity of the air in contact with the object; indoor air quality and pollu- tant concentration; electromagnetic radiations coming from sources of natural and articial light. Their synergic effect can af- fect degradation too 10.As regards in particular thermal-hygrometric conditions of the air in contact with the artworks, their values are important but over all their sudden change represents the greatest risk to conser- vation. In actual fact, even with optimal values of T and RH, rapid changes of such conditions can cause degenerative processes: thus, stability in the time of indoor parameters must be obtained. Tech- nical literature 2,11 insists above all on the control of RH, whose changes can cause irreversible damage, in particular for hygro- scopic materials. Hygrometric control has to ensure that the va- pour transfer between the indoor environment and the materials is such as to reduce the risk of damage. RH, in fact, affects the changes of dimension and form of organic materials capable of absorbing water, e.g. wood, ivory, leather, paper, etc., which will swell when RH rises and shrink when it falls, with consequent weight changes, deformation and cracks 12. In addition, intersti- tial condensation inside the materials, together with the pollutantsin the air (CO2, NOx, SO2, O3, etc), gives rise to aggressive solutions that cause corrosion of the metals, discolouring of the drawings on cottons, axes, wools, silks, and weakening of organic bres (tex- tiles and paper), particularly in the presence of light 13.While low temperatures are not particularly dangerous for the artworks, on increasing temperature degradation processes accel- erate, with consequent risk for conservation. Moreover, RH exceed- ing 65% associated to T in excess of 20 C increases the development of mycete colonies and accelerates the life cycles of damaging in- sects 4.The conservation of the artworks is not easy since, even though the best conditions for the conservation of the different categories of works are known, quantitative information is scarce as to the acceleration that the various degradation phenomena may have due to changes from ambient design conditions 14. An interdisci- plinary investigation by several professional gures (curator, HVAC designerinstallermanager) is necessary to determine the optimal values of microclimatic parameters, based on the conservation state of the object, in relation to its climatic history and previous parameter values. For museum air-conditioning, the recommended thermal-hygrometric parameters for the conservation of the vari- ous materials have placed priority on human comfort and are stric- ter as regards both RH and T. Due to the necessity of guaranteeing artwork conservation and, if possible, thermal comfort for visitors too, moderate indoor conditions are preferable in exhibition areas,with stable values of T and RH. Optimal values of thermal-hygro- metric parameters for museum applications are reported in the international standards 1 and in national regulations 15. Rec- ommended values mainly concern air T and RH, as well as their maximum admitted variation.A suitable microclimate for the conservation 16 has to be cho- sen taking into account both the direct impact that it has on the materials of the objects and the indirect impact it has in creating a favourable habitat for biological degradation and undesirable chemical reactions, especially in the presence of atmospheric pollutants.In general, it is possible to identify the following criteria 16: If an object is in a favourable microclimate and there are no deg- radation processes acting, the object must be kept in such envi- ronmental conditions. The original microclimate can be improved removing or attenu- ating the perturbing causes, such as day cycles, uctuations, quick transitions 1. If necessary, the microclimate of an object must be changed on the basis of specic studies and the transition to the new condi- tions must be very slow. In absence of knowledge of the history of an object, the choice of the microclimate has to be made on the basis of its chemical physical characteristics.Also passive methods play an important role, such as interven- tions on the building envelopes in order to increase inertia, opti- mize heat and mass exchanges, reduce the incidence of luminous radiation.2.2. HVAC systems for museumsThe HVAC system must guarantee the indoor design microcli- matic conditions, controlling the transient phenomena and, at the same time, it should be well integrated in the building structure.In particular, the fraction of the internal load due to the occupancy can create problems for the time stability of the thermal-hygrometric conditions: in fact, this is an impulsive and not attenuated load be- cause of the high occupancy variation. Such discontinuous ow of people can cause signicant and sudden changes of the microclimatic parameters in the museum environment; so the reaction of the HVAC system must be quick and effective in order to restore the de- sign values of the thermal-hygrometric conditions for the conserva- tion. On the contrary, the thermal load related to outdoor conditions changes more slowly because in many buildings destined to muse- ums the thermalphysical characteristics of the envelopes (heavy structure) induce a high thermal inertia and consequently an atten- uation of the instantaneous thermal gains: therefore, the HVAC system is generally able to keep the design conditions without sig- nicant indoor changes.In order to keep the ambient conditions stable in the time, it is necessary that HVAC systems remain constantly operating, as678F. Ascione et al. / Applied Thermal Engineering 29 (2009) 676686Fig. 1. Plans of the ground oor, the rst oor and the roof top with the skylights.regards the exhibition spaces and the stocks; therefore, system typologies which allow considerable energy saving should be used. Moreover, in order to reduce the transient phenomenon period,Table 1Main characteristics and design conditions for the simulated exhibition roomArea272 m2Inltration0 exchanges/hminimum airow rate values should vary from 6 to 8 air changesper hour 1,17; thus, a constant-volume HVAC system is usually preferred.1In the HVAC system design for the exhibition spaces, the diffu- sion of the air in the room must be carefully considered in order to avoid the formation of stagnant zones and to realize a low speed air circulation.Volume1360 m3Light equipment design thermal loadIndoor T*Outdoor ventilationairow rateIndoor RH50 5%Skylight thermal transmittance Skylight shading factor20 W/m26 L/s (21 m3/h) per person2.00 W/m2 K0.764Other aspects that inuence HVAC system choice are mainte-nance access and risk of the collection disruptions due to leaks from overhead or decentralised equipment and from water orOutdoor climatic dataOccupancy leveldesign dataRome TRY data 70.3 person/m2(total: 82 persons)Exterior wall thermal transmittanceRoof thermaltransmittance0.48 W/m2 K0.37 W/m2 Ksteam pipes over and within collection areas.In order to achieve the project goals before described, all-air systems are generally preferred: a centralised air-handling unitOccupantmetabolic rate1.5 met/personElectrical utility rate0.11 /kW hGas utility rate0.55 /Sm3keeps equipment, maintenance and monitoring at some distance from the collections.The adoption of adsorption dehumidication systems can be useful too, as it allows the reduction of the humidity especially if high latent loads occur. This dehumidication type is better also in terms of hygiene as the absence of condensed water greatly re- duces the presence of bacteria, fungi and microbes.*Annual xed set point (22 1 C), or seasonal set point with T ranging gradually from 21 1 C to 23 1 C.At the ground oor, the administrative, commercial and accommo- dation areas are located, as well as a stock room. Upstairs there are three exhibition rooms, characterized by an inner height of 5 m.23. Case study and HVAC system analysisThe main exhibition room has a total inside area of 272 mvolume of 1360 m3.and aThe simulated modern museum (Fig. 1) covers a total area ofParticular feature of the exhibition space is the presence of sky-21200 m2, distributed on two oors, each of 600 m2 approximately.lights,typical in various museums (e.g. Guggenheim in NY and AraPacis in Rome). It can be noted that the use of natural light could rep-1 In the case study after reported, the constant airow rate is equivalent to around 6 air changes per hour.2 The skylights are characterized by a transparent surface in acrylic polycarbonate, with a thermal break aluminum frame.F. Ascione et al. / Applied Thermal Engineering 29 (2009) 676686679Fig. 2. Average monthly outdoor temperature and relative humidity (derived from TRY data) for Rome.Fig. 3. Hourly occupancy schedule for the museum examined.resent a net energy penalty and a risk for the collections 1. So, par- ticular screens have been considered over skylights.The attention has been focused on the exhibition space, because of the interest both in the artwork conservation and in the comfort for occupants. The design conditions 18,19 and some building thermalphysical parameters are reported in Table 1. In Fig. 2 aver- age monthly outdoor temperature and relative humidity (derived from TRY data) are reported for Rome. As regards indoor T values, ASHRAE indications 1 relative to the admitted uctuation classes for indoor T and RH for the museum environment have been taken into account. Thus, two different thermostat scheduling strategies have been considered: an annual xed set point (22 1 C), strategy sometimes adopted in the museums, or a seasonal set point, with T ranging from 21 1 C (DecemberMarch) to 22 1 C (AprilMay and OctoberNovember) and 23 1 C (JuneSeptember).Moreover, a scheduling of internal lighting has been considered, as well as an annual scheduling of hourly occupancy for a typical museum (Fig. 3). The occupancy schedule is the same throughout the year; the museum is open every day from 9 a.m. to 8 p.m., ex- cept on Tuesday (closed).The HVAC system chosen for the analyzed case study is an all- air system with constant air volume (CAV) for single zone (Fig. 4a): this simple conguration has been used as reference in the comparison with the systems reported in Fig. 4bd, below described. System with desiccant module (Fig. 4b), indicated as CAV-DW in the gures, is characterized by adsorption dehumidication obtained with a desiccant wheel 20. The desiccant module pre- sents also a regeneration coil, a sensible heat exchanger and an evaporative cooler.3 System with total heat recovery (Fig. 4c), indicated as CAV-ERV in the gures, presents an enthalpy wheel between the ventila- tion air and the exhaust air, considering a purge ow rate of 10% of the total ventilation ow rate in according to literature indi- cations 3. The algorithm used to model the heat transfer takes into account the hourly variability of the effectiveness as a func- tion of the make-up and exhaust airows. The control sequences for the heat exchanger 6 have been chosen in order to maxi- mize the energy saving. Thus, the recovery will operate only when the enthalpy difference between the outside and exhaust air is at least 2325 kJ/kg (= 1 Btu/Lb). Moreover, a bypass out- door air strategy has been considered in order to both control the outlet air conditions after the mixing and to compensate for the overheating and overcooling effects. System with outdoor airow rate variation (Fig. 4d), indicated as CAV-DCV in the gures, is charact