Solar cooling and heating plants: An energy and economic analysis of liquid sensible vs phase change material (PCM) heat storage

Accepted Manuscript Solar Cooling And Heating Plants: An Energy And Economic Analysis Of liquid Sensible vs phase change material (PCM) Heat Storage M. Noro, R.M. Lazzarin, F. Busato PII:

S0140-7007(13)00200-4

DOI:

10.1016/j.ijrefrig.2013.07.022

Reference:

JIJR 2575

To appear in:

International Journal of Refrigeration

Received Date: 26 March 2013 Revised Date:

22 July 2013

Accepted Date: 25 July 2013

Please cite this article as: Noro, M., Lazzarin, R.M., Busato, F., Solar Cooling And Heating Plants: An Energy And Economic Analysis Of liquid Sensible vs phase change material (PCM) Heat Storage, International Journal of Refrigeration (2013), doi: 10.1016/j.ijrefrig.2013.07.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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SOLAR COOLING AND HEATING PLANTS: AN ENERGY AND ECONOMIC ANALYSIS OF LIQUID SENSIBLE VS PHASE CHANGE MATERIAL (PCM) HEAT STORAGE

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M. Noro* ([email protected]), R.M. Lazzarin ([email protected]), F. Busato ([email protected]) Department of Management and Engineering - University of Padova - Stradella S. Nicola, 3 - 36100 Vicenza – ITALY

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*Corresponding author. Tel.: +39 0444 998704; Fax: +39 0444 998884

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e-mail address: [email protected]

ABSTRACT

A key factor for the energy optimization of a solar heating/cooling plant is the design of the heat storage. Latent heat storage system using phase change materials (PCMs) is an effective way of storing thermal energy and takes advantage from the high-energy storage density and the isothermal nature of the storage process. It is interesting to evaluate the potential of integrated solar absorption cooling and heating systems

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with sensible versus PCMs heat storage tanks. Dynamic transient simulations by Trnsys tool were used as a basis for assessment for a typical offices building and solar heating and cooling plant application sited near Rome. An optimization of the storage capacity from both energy and economic point of view has been

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performed, considering the tanks on both the sides of the plant (the one coupled to the solar field, the hot storage, and the other coupled to the absorption chiller, the cold storage). Different cases have been

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simulated: both tanks modeled as sensible (water) storage, only hot side tank modeled as PCM storage and only cold side tank modeled as PCM storage. In the second case, two different sub-cases have been further considered: “hot” (temperature of fusion 89 °C) and “warm” (temperature of fusion 44 °C) PCM heat storage. Results indicate that the solution that features the lowest global primary energy consumption and the highest solar ratio provide a 3000 l “warm” tank filled with PCM melting at 44 °C and a 2000 l “cold” sensible (water) storage.

KEYWORDS

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Absorption chiller – solar collector – solar cooling – solar heating – Phase Change Materials

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1. INTRODUCTION

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NOMENCLATURE Cool tank cooling energy stored (J) Cool_tank_Loss cooling energy lost by cold tank (thermal gain) (J) Diss energy dissipated by dry-cooler (J) DPB Discounted PayBack period (y) E_tot Total electrical energy delivered to the plant from the grid (kWh, J) FE01 useful thermal energy collected by solar field (J) FE02 useful solar thermal energy to solar tank (J) FE03 thermal energy integrated by boiler (direct to the building in heating mode, to the generator of the absorption chiller in cooling mode) (J) FE07 thermal/cooling building needs (J) FE06 cooling energy produce by chiller (J) FE05 thermal energy supplied to the generator of the absorption chiller in cooling mode (J) FE04 thermal energy dissipated by cooling tower (J) G_coll global irradiance (for non-concentrating collectors) or beam irradiance (for concentrating collectors) on the plane of thermal collectors (J) HVAC Heating, Ventilation, Air Conditioning i Interest rate Loss_coll energy lost by solar thermal collectors (J) NCF Net Cash Flow (€) NPW Net Present Worth (€) P Initial economic extra-investment for solar heating/cooling plant with respect to a traditional plant (€) PCM Phase Change Material PE Primary Energy (J) PES Primary Energy Saving (J) S Yearly economic saving of the solar heating/cooling plant with respect to a traditional plant (€/y) SC01_Loss energy lost by SC01 heat exchanger (J) t year Tank_to_load useful thermal energy supplied by solar tank (to the building in heating mode, to the generator of the absorption chiller in cooling mode) (J) TRY Test Reference Year

Data published by ESTIF (European Solar Thermal Industry Federation) indicates that, even under such difficult economic conditions due to the financial and economic crisis, solar heating and cooling has proved to be a driver in the European energy strategy, especially towards achieving the 2020 targets. With almost 2.6 GWth installed in 2011 (3.7 Mm2 solar collectors, the same data of 2010), the total installed capacity in Europe is (at the end of 2011) 26.3 GWth (37 Mm2) (ESTIF, 2012). In Italy small size systems (less than 30 m2) are mainly spread for domestic hot water production in residential buildings. There are also various combi-systems (domestic hot water + space heating) mainly

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located in the Northern part of the country. In the Southern part the use of combi-systems may be not convenient from both energy and economic point of view because of the low buildings thermal loads and the high solar radiation. This scenario changes dramatically when considering solar cooling that is the use of

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thermal collectors in summer to satisfy cooling loads of the buildings. One of the main aspects of solar systems is storage. Storage density, in terms of the amount of energy per unit of volume or mass, is an important issue for applications in order to optimize solar ratio (how much of the solar radiation is useful for the heating/cooling purposes), efficiency of the appliances (solar thermal

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collectors and absorption chillers) and room consumption. For these reasons, it is worth to investigate the possibility of using Phase Change Materials (PCM) in solar system applications. The potential of PCMs is to

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increase the energy density of small sized water storage tanks, reducing solar storage volume for a given solar fraction or increasing the solar fraction for a given available volume (Medrano et al., 2009). A further advantage is that heat storage and delivery normally occur over a fairly narrow temperature range (the transition zone) (Paris et al., 1993).

(Chidambaram et al., 2011) report a review of several researches developed during the last years concerning

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PCM heat storage, all related to the performances of the storage itself rather than of the system. Oró reports a study on a solar cooling and refrigeration plant in which different PCM have been tested under different conditions (Oró et al., 2012). No study concerning energy and economic optimization of a solar heating and cooling plant with PCM vs sensible heat storage is available to the best of the authors’ knowledge.

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This paper reports on the study of a solar heating/cooling plant coupled to a sensible versus PCM thermal storage to face the thermal and cooling loads of a building. Annual simulations have been carried out based

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on an existing building (offices) sited at ENEA Casaccia Research Centre (near Rome, Italy) (ENEA is the Italian National Agency for New Technologies, Energy and Sustainable Economic Development; Casaccia is the largest Research Centre in Italy). The solar cooling and heating plant presents two water storage tanks, one coupled to the solar field and another coupled to the absorption chiller. The former operates as a “hot” storage tank during summer cooling season (driving an absorption chiller) and as a “warm” storage tank during winter heating season; the latter obviously operates only as a “cold” tank only during cooling season. So two issues have been faced with the aim of minimizing the primary energy consumption and maximizing the economic advantage with respect to a traditional heating/cooling plant: the evaluation of the best size of

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the tanks and the optimum temperature range and so the solid-liquid phase change temperature of the PCM replacing water for designing one of the two storages as a “PCM storage”. It is suitable to introduce the analysis by a brief introduction of the main characteristics of the thermal

2. HEAT STORAGE IN SOLAR COOLING AND HEATING PLANTS

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storage and the proper PCMs in solar plants.

Heat storage in solar (cooling) plants is an essential issue. It is possible to think of thermal storage in the hot

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and/or in the cold side of the plant. The former allows the storage of hot water from the collectors (and from the auxiliary heater) to be supplied to the generator of the absorption chiller (in cooling mode) or directly to

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the users (in heating mode). The latter allows the storage of cold water produced by the absorption chiller to be supplied to the cooling terminals inside the building. It is usual to identify the three situations just described as, respectively, “hot”, “warm” and “cold” storage because of the different temperature ranges. Typically, a hot tank may work at 80-90 °C, a warm tank at 40-50 °C and a cold tank at 7-15 °C. While heat storages in the hot side of solar plants are always present because of heating and/or domestic hot

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water production, cold storages are justified in bigger size plants. Cold storages are used not only to get economic advantages from the electricity tariffs (in case of electric compression chiller) depending on the time-of-the-day but also to lower cooling power installed and to allow more continuous operation of the chiller (Lazzarin, 2007).

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Speaking about the hot tank, the thermal level should be as high as reasonably possible because the absorption chiller can produce a higher cooling capacity even with high values of the cooling water produced

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by the cooling tower. However, in some periods of the day or the season the cooling load can be lower, so that a lower cooling capacity is needed. In these cases the use of a double tank storage can be suggested, operating at two different temperature levels (Lazzarin, 2007). To avoid an unfavourable on-off (discontinuous) operation of the absorption chiller when the cooling demand lowers, a cold storage may be recommended which can allow long continuous working periods of the machine with long term performance closer to nominal. Moreover a lower capacity chiller can be selected which can operate in more favourable temperature fields, as the peaks of demand can be met by the joint contribution of the chiller and of the cold storage.

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3. HEAT STORAGE BY PHASE CHANGE MATERIALS IN SOLAR COOLING PLANTS Generally speaking, energy storages may be “short-term” or “long-term” (a few hours or days in the first

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case, a few months or even the whole season in the second). Here we consider only the former. Another possible classification is between sensible and latent (PCM) heat storage. The sensible storage of heat is directly connected to thermal capacity, then to temperature rise of the substance (typically water). In the second case the heat is mainly stored in the phase change process (at a quite constant temperature) and it is

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directly connected to the latent heat of the substance.

As depicted in Figure 1, the phase change process takes place in different modes: solid-solid, liquid-gas and

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solid-liquid. In the first case heat is stored by transition between different kinds of crystallization forms. It is characterized by a relative low latent heat (more promising materials are solid organic solutions such as Li2SO4 and KHF2) (Sharma et al., 2009). For liquid-gas systems latent heat is very high, but there are some problems in storage control due to the high volume variations during phase change. The most widespread are the solid-liquid PCMs, that have a limited volume variation during latent heat exchange (generally less than

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10%) and a fairly high melting latent heat. In the following analysis we will refer just to solid-liquid PCMs. Possible applications of PCMs are (Sharma et al., 2009): −

implementation in gypsum board, plaster, concrete or other wall covering material being part of the building structure to enhance the thermal energy storage capacity, with main utilization in peak load

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shifting (and shaving) and solar energy (Khudhair and Farid, 2004). In this application typical operating temperature is 22-25 °C but it can vary as a function of climate and heating/cooling loads; cold storage for cooling plants (operating temperature 5-18 °C) (He and Setterwall, 2002);

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warm storage for heating plants (45-60 °C) (Farid et al., 2004);

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hot storage for solar cooling and heating (> 80 °C) (Sharma et al., 2009) (Bruno, 2005)

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(Vakilaltojjar, 2000).

INSERT HERE FIGURE 1

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Concerning the last three types, in (Busato et al., 2012a) the authors found that energy performances of PCM storages in solar cooling/heating plants are strictly connected to the temperature level of the phase change,

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and so to the solar collectors field size and characteristics, the storage tank volume and obviously to the supply temperature to the heating appliances. This is because PCMs perform better than sensible heat storage only during periods when mean temperature of the storage is around melting temperature of the selected PCM. For this reason the study here reported depicts an energy and economic comparison of the three types

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of PCM storages (cold, warm and hot) with respect to the sensible storage. The study concerns the annual air

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conditioning of an existing office building coupled to a solar cooling and heating plant (see next section 4).

3.1. Characteristics of PCMs for solar cooling/heating plants

There are a lot of PCMs but none have all the characteristics necessary to completely fulfill the application here considered. The main properties are (Sharma et al., 2009): −

thermo-physical (latent heat of transition and thermal conductivity should be high, density and

storage volume); −

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volume variation during phase-transition should be respectively high and low in order to minimize

kinetic and chemical (supercooling should be limited to a few degrees, materials should have longterm chemical stability, compatibility with materials of construction, no toxicity, no fire hazard); economics (low cost and large-scale availability of the phase change materials are also very important).

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2011): −

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The main advantages and drawbacks of PCM versus water sensible heat storages are (Chidambaram et al.,

the possibility to reduce the tank volume for a given amount of energy stored, that is true only if storage is operated in a very narrow temperature range around phase-transition temperature;

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less on-off cycles of auxiliary heaters (for plants with storage in the hot side) and chillers (for plants with storage in the cold side);

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higher investment costs;

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higher risks due to leak of stability and erosion of material encapsulating PCMs. 6

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INSERT HERE FIGURE 2

Phase Change Materials can be classified in organics and inorganics (Figure 2). Eutectics are a minimummelting composition of two or more components, each of which melts and freeze congruently forming a

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mixture of the component crystals during crystallization; eutectics nearly always melt and freeze without segregation For this reason they are a promising type of PCM for the future, even if actually they are less

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diffused than the other groups. Considering real applications in thermal energy store, the most widespread materials are paraffins (organics), hydrated salts (inorganics) and fatty acids (organics). In cold storage ice water is quite used as well. Here a brief summary of the main types of PCM is reported, whereas in recent literature it is possible to find more examples and information (Cabeza et al., 2011) (Sharma et al., 2009)

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(Osterman et al., 2012) (Fan and Khodadadi, 2011) (Agyenim et al., 2010) (Baetens et al., 2010).

3.2. Organic PCMs

They can melt and solidify a lot of times without phase segregation and consequent degradation of their

main groups are:

paraffin waxes: they consists of a mixture of mostly straight chain n-alkanes CH3–(CH2)–CH3 in

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−

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latent melting heat, they crystallize with little or no supercooling and usually non-corrosiveness. The two

which the crystallization of the (CH3)- chain releases a large amount of latent heat. Due to cost consideration, however, only technical grade paraffins may be used as PCMs in latent heat storage systems. Paraffin is safe, reliable, predictable, less expensive, non-corrosive and available in a large temperature range (5-80 °C) (Sharma et al., 2009) (Himran et al., 1994); −

non-paraffin organic: they are the most numerous of the phase change materials with highly varied properties. A number of esters, fatty acids, alcohols and glycols suitable for energy storage have been identified (Buddhi and Sawhney, 1994) (Abhat et al., 1981). Some of the main features of these

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organic materials are: high heat of fusion, inflammability, low thermal conductivity, low flash points, and instability at high temperatures.

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3.3. Inorganic PCMs These phase change materials do not supercool appreciably and their melting enthalpies do not degrade with cycling. The two main types are: −

salt hydrate. They are alloys of inorganic salts (AB) and n kmol of water forming a typical

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crystalline solid of general formula ABnH2O whose solid–liquid transition is actually a dehydration and hydration of the salt. They have been extensively studied in heat storage applications because of

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their positive characteristics: high latent heat of fusion per unit volume, relatively high thermal conductivity (almost double of the paraffin’s), they are not very corrosive and compatible with plastics. Some relevant disadvantages are incongruent melting and supercooling, which can be tackled in different ways (adding thickening agents, by mechanical stirring, by encapsulating the PCM to reduce separation, etc.) (Sharma et al., 2009);

metallics. This category includes the low melting metals and metal eutectics. They are scarcely used

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in heat storage applications because of their low melting enthalpy per unit weight, even if they have high melting enthalpy per unit volume and high thermal conductivity.

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3.4. PCMs containment

Containment of PCM is worth in order to contain the material in liquid and solid phases, to prevent its

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possible variation in chemical composition by interaction with environment, to increase its compatibility with other materials of the storage, to increase its handiness and to provide suitable surface for heat transfer. Types of containment studied are bulk storage in tank heat exchangers, macro-encapsulation and microencapsulation. The main characteristic of PCM bulk systems is the need for a more extensive heat transfer than that found in non-PCM tanks because the heat storage density of the PCM is higher compared to other storage media. The different approaches extensively used are inserting fins or using high conductivity particles, metal structures, fibers in the PCM side, direct contact heat exchangers or rolling cylinder method (Felix Regin et al., 2008).

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The two other possibilities are macro and micro encapsulating (Cabeza et al., 2011). Macro encapsulating consists in including PCM in some tube, sphere, panel, cylinder or other, and is the most widespread. The choice of the material (plastics or metallics - aluminium or steel) and the geometry affect thermal

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performance of the heat storage. Micro encapsulating consists of micro-sphere (diameter less than 1 mm) of PCM encapsulated in a very thin and high molecular weight polymer. The spheres are then incorporated in some compatible material.

It is very important to evaluate the performance of the heat storage after a lot of load-unload cycles in order

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to verify the stability and duration of the PCM-container system.

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3.5. Commercial PCMs

In 1990 only 12 companies were producing PCMs for thermal energy storage, all in the United States. In recent years a lot of new companies are in the market from all over the world, even if a complete list or reference web site of all the companies involved are still missing to the best authors’ knowledge. “Cristopia” from France, “TEAP Energy” from Australia, “Rubitherm GmbH” from Germany, “EPS Ltd” from UK are

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some examples from (Kenisarin and Mahkamov, 2007). Table 1 reports some products from one of these

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companies.

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INSERT HERE TABLE 1

One of the most important limits to the diffusion of PCMs is their cost. For the experience of the authors, the costs reported in Table 1 may be raised till 4 € kg-1. The main objective of next section is to assess the energetic advantages of PCM heat storage in order to justify the greater cost with respect to water sensible heat storage.

4. SIMULATIONS OF A SOLAR COOLING AND HEATING PLANT WITH SENSIBLE VS PCM HEAT STORAGES 9

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4.1. Description of the simulation model The case study concerns an existing real building, the F92 sited at ENEA Casaccia Research Centre near Rome (Italy) that houses the “Scuola delle energie” (“Energies School”); it is developed in three storeys

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(basement, ground floor and first floor, the lasts being identical) for a volume of 620 m3 and a total floor surface of 230 m2. The building/plant system has been modeled in Trnsys on the basis of transparent and opaque surfaces characteristics, internal gains and thermal/cooling plant schedules data supplied by ENEA (Busato et al., 2012b). Briefly, the thermal transmittance of the external walls, roof and windows are,

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respectively, 0.72, 0.52 and 2.8 W m-2 K-1. The winter diurnal temperature set point is 20 °C, whereas an attenuation down to 16 °C is supposed when nobody is present during the night time. The summer

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temperature is set at 26 °C, let rising up to 28 °C following the schedule. No air relative humidity control has been foreseen. The heating and cooling loads have been calculated on the basis of Rome Test Reference Year (AA.VV., 1985) with a time step of one hour. For the heating season (1st Nov – 15th Apr) no internal heat gains have been considered, resulting in a thermal load of 19.2 kW and thermal needs of 52.3 GJ (63 kWh m-2 y-1); for the cooling season (1st June – 15th Sept) internal heat gains have been considered, resulting

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in a cooling load of 15.9 kW and cooling needs of 35 GJ. As regards the domestic hot water, due to the use of the building (offices and teaching rooms) the daily demand is very small and it is satisfied by means of

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electric resistances.

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INSERT HERE FIGURE 3

The HVAC system at F92 building is made of (Figure 3): −

the heating/cooling plant, consisting of a single stage absorption chiller (coupled to a cooling tower) and a backup/integration natural gas boiler;

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the solar field (five parallel arrays of three solar thermal collectors connected in series);

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a 90 m long network (steel EN 10204/31 B double pipe with rigid polyurethane foam coating) connecting the heating/cooling plant to the indoor substation distributing the heat/cool to the floors; 10

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heating/cooling units: every floor is supplied by an independent circuit. The basement is supplied by fan-coil units only, the other two floors by both radiant floors and fan-coils. During heating season, radiant floors are supplied by the solar heat storage only, while the fan-coils by the boiler only.

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During cooling season, both radiant floors and fan-coils can operate, following the control strategy described in next section 4.2.

The solar field is connected to the heating plant by a 1500 l sensible (water) tank, while the absorption chiller is connected to the cooling plant by a 1000 l sensible (water) tank. All the technical features of the

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(Calabrese and Fanchiotti, 2012) and briefly reported in Table 2.

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equipment of the modeled plant have been referred to the real ones described in (Busato et al., 2012b) and

INSERT HERE TABLE 2

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4.2. Control strategy of the plant

Simulations have been run following the real control strategy of the solar cooling/heating plant whose main objective is to minimize the fossil primary energy consumption of the integration boiler; the operation is here

Heating mode.

As stated in previous section, the solar tank is used only to feed the low temperature radiant floor units,

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briefly described (refer to Figure 3):

while during the start up of the heating (first hours in the morning or after a long period of off plant, when a high thermal power is demanded for a short time to heat the whole water content of the plant) only the fan-coils fed by the boiler are used. So boiler and P04 pumps start their operation when both solar storage temperature T07≤39 °C and network inlet temperature T20≤54 °C, and stop when both T07≥41 °C and T20≥56 °C. When radiant floor units are in operation fed by solar energy, V02 three ways mixing valve keeps network inlet temperature T22=45 °C. In order to not dissipate the solar energy stored in the hot tank during the start up phases of the plant, a dedicated logic has been implemented at

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the first operation of the solar storage tank + radiant floors to keep the return temperature (T19) higher than 27 °C. Solar collectors field (that is pump P01A) starts its operation when illumination is above 1050 lux (and

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stops when below 950 lux). Solar storage (1500 l) is loaded by the SC01 plate heat exchanger. Modulation of P01 pumps is a function of T01 temperature (solar collectors outlet) in order to keep it always higher than 30 °C. Pump P02A starts its operation when T01-T07>3.5 °C (water at the outlet of solar collectors is sufficiently hotter than heat storage) and stops when T01-T07≤2.5 °C. Furthermore, if

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T01≥81 °C P02B starts its operation in order to increase the thermal power exchanged to the SC01 and so to decrease T01 itself; if instead T01≥91 °C P01B starts its operation. The two pumps stop when

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respectively T01≤79 °C and T01≤89 °C. In order to avoid boiling point inside the hot tank, the drycooler starts its operation when T07≥96 °C (P02 stop in order do not dissipate solar energy stored in the solar tank) and stops when T07<90 °C.

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Cooling mode.

In this case solar energy is used to supply the generator of the absorption chiller in order to produce cooling water stored in the cold tank (1000 l). The operation of the solar field is the same of the heating

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INSERT HERE FIGURE 4

mode. V02 three ways mixing valve keeps inlet generator temperature T22≤88 °C. In the case solar energy is not enough to operate the chiller (T07≤79 °C) the boiler starts its operation, but only if T11A≥70 °C.

The operation of the absorption chiller (and the coupled cooling tower) is driven by the cold tank temperature T18, whose set point is decreased from 18 °C to 7 °C as a function of the indoor air temperature of ground and first floors (linearly from 21 to 26 °C). The use of cooling radiant surface or fan-coils depends on the indoor ambient temperature, respectively lower and higher than 26 °C (in the first case the minimum inlet temperature of water being 14 °C). 12

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Other details of the operating logic are reported in (Calabrese and Fanchiotti, 2012). Some relevant controls have been done during simulations, in order to check the energy conservation law (Figure 4) and the correct

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temperatures of the single subsystems and of the whole system.

4.3. The scenarios

Simulations have been carried out on the previously described system comparing different scenarios: a) Solar plant with both tanks modeled as sensible (water) storage (using type 60 in Trnsys);

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b) Solar plant with only the hot side tank modeled as PCM storage (type 860, (Bony and Citherlet, 2007) (Streicher, 2008));

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c) Solar plant with only the cold side tank modeled as PCM storage.

In scenario b) two different cases have been further considered: “hot” PCM heat storage (the temperature of fusion is 89 °C, named S89) and “warm” PCM heat storage (temperature of fusion 44 °C, named S44). The PCMs considered are salts hydrate of the Phase Change Material Product Limited (UK), whose main characteristics are depicted in Table 3. The type 860 has to be set up with many parameters, the mains being: the temperature-enthalpy characteristic of the PCM: Figure 5 reports the curves for the three materials

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−

considered for the simulations; −

type and dimensions of the encapsulation: the PCMs have been considered inserted in HDPE plastic tubes (50 mm diameter, 1 m length);

thermal conductivity and specific heat capacity both at liquid and solid phase, latent fusion heat (Table 3);

parameters of type 860 concerning other issues like hysteresis and subcooling of the materials have been

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set up at their default values as no specific data were available from the PCMs supplier.

INSERT HERE TABLE 3

INSERT HERE FIGURE 5

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The three scenarios have been compared in order to investigate the best solution from the energy (minimum fossil primary energy consumption) and the economic (maximum Net Present Worth, NPW, minimum

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Discounted PayBack period, DPB) points of view. It is worth to remember that the NPW of a time series of net cash flows, both incoming and outgoing, is defined as the sum of the present values of the individual cash flows, given the interest rate (3% in this study) and the period of the analysis (15 years). NPW is a central tool in discounted cash flow analysis, as DPB is, that is the period of time required for the return on the

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investment in solar cooling plants to “repay” (by the economic savings S) the sum of the original extrainvestment P with respect to a traditional plant:

t =0

NCFt (1 + 0.03) t

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NPW = ∑

S   log  S − P ⋅ 0.03   DPB = log(1 + 0.03)

(1)

(2)

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5. ENERGY AND ECONOMIC ANALYSIS: RESULTS AND DISCUSSION 5.1. Energy performance analysis

Data measured by the different energy meters (Figure 3) for the first period of heating operation (from 9th February till 15th April 2012) and for the cooling season (1st June – 15th September 2012) were available to

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the Authors with a time step of one hour. So first simulations were run in order to calibrate the model: Figure 6 reports a comparison between measured and simulated data for the main quantities of energy: useful solar

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energy produced by the solar collectors (FE02), useful solar energy to the user (the heating units inside the building in winter, the generator of the absorption chiller in summer, Tank_to_load), boiler integration energy (FE03), building energy needs (FE07) and cooling energy produced by the absorption chiller in summer (FE06). Some mismatching is present: it is due in part to the difference between real climatic conditions and the TRY used in simulations and mainly to the initial different setting of the plant control logic (that has been set up as previously described only after the middle of March 2012, for details see (Busato et al., 2012b) and (Calabrese and Fanchiotti, 2012)).

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INSERT HERE FIGURES 6a AND 6b

Once the model has been tested and calibrated by comparison with measured data, simulations were run in order to get energy results. For sake of brevity, Figure 7 and Figure 8 report the comparison between the four types of storage (sensible, hot, warm and cold) for three different tank capacity couples in terms of annual

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energy results only. The comparison has been done in terms of (refer also to Figure 4): energy balances between the main quantities:

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(3)

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 FE 07 heating season   ((((G _ coll − Loss _ coll ) − Diss ) − SC 01 _ Loss ) − Tank _ losses ) + FE 03 =  FE 05 cooling season 14444244443 + ( FE 07 + Cool_tank_ Loss ) = FE 04 q _ coll  14444244443 144444 42444444 3 FE 06  144444FE 401 44 42444444444 3 FE 02 144444444444424444444444443 Tank _ to _ load

fossil Primary Energy (PE) consumption, calculated taking into account rated natural gas boiler efficiency (90.1%) and a primary energy factor of 0.46 for electricity consumed by pumps, fans, etc.

PE =

(4)

Primary Energy Saving (PES), defined as the useful solar energy that it should be produced by the

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−

FE 03 E _ tot + 0.91 0.46

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(E_tot):

natural gas boiler in a “traditional” (no-solar) plant PES =

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Tank _ to _ load 0.91

(5)

solar ratio, calculated as the ratio between the useful solar energy and the total thermal energy (from solar and boiler) required to satisfy thermal/cooling needs : Solar ratio =

Tank _ to _ load Tank _ to _ load + FE 03

(6)

The best solution, from the energy point of view, is given by the 3000-2000 l tanks capacity, the former filled with PCM S44 (that is a “warm” tank) and the latter being a sensible storage (water). It features the 15

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lowest integration boiler energy need (FE03) and one of the lowest thermal energy dissipated by the drycooler (Figure 7); at the same time it features the lowest global PE and the highest PES, solar ratio and PER (Figure 8). Figures highlight that in any case increasing the tanks capacity has a positive effect. Because

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operating temperature of the storage depends on the solar collectors area, volume of the storage tank, kind and entity of the thermal loads to be faced, the PCM that performs better in this case is S44. In fact heating loads are higher than cooling loads (Figure 6), that is mean operating temperature of the storage is near melting temperature of this PCM for long during the year. Figure 9 reports the operating temperatures (at the

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upper part, at the bottom and the mean temperature of the tank) of a typical week of November; it is quite

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evident the “plateau” of the mean temperature around the melting temperature of the PCM.

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5.2. Economic performance analysis

From the economic point of view, a comparison between a “traditional” heating/cooling plant (natural gas boiler, absorption chiller and sensible water storages of the same characteristics of the existing plant) and the solar plant (with both sensible and PCMs storages) has been carried out. The main hypotheses concerning the analysis are reported in Table 4 to Table 6.

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From the economic point of view, the best solution is the solar plant with sensible heat and cool storages (Figure 10). Anyway, the use of PCM allows an economic advantage as well with respect to the “traditional” plant; the “warm” tank (PCM S44) gives the best economic performance in this case. It is worth to highlight that in this case the economic advantage is given by the solar plant, not by the PCM storage: comparing the solar plant with sensible vs PCMs storages, NPWs are always negative and DPB not computable. The greater

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investment cost of PCM storage is not justified in the case here considered; it could be for the case “2000 l with PCM S44 tank – 1000 l sensible cold tank” when considering, for example, natural gas cost higher by

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30% or PCM tank investment cost lower by 70%.

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6. CONCLUSIONS

The study reports the energy and economic analysis of a solar plant for heating and cooling of an offices building, with the main objective of defining the size and the type (sensible vs PCM) of the thermal energy storages placed in the hot and in the cold side of the plant. It is worth to stress that the operating temperature of the storage depends on the solar collectors area, volume of the storage tank, kind and entity of the thermal loads to be faced. Only a transient simulation allows a reliable comparison between different layouts and 17

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sizes of system components. The best solution, from the energy point of view, is given by the 3000-2000 l tanks capacity, the former filled with PCM S44 (that is a “warm” tank) and the latter being a sensible storage (water). It features the higher primary energy saving and solar ratio between the different scenarios

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considered because mean operating temperature of the storage is near melting temperature of this PCM for longer during the year. Is confirmed the main result that the authors found in previous study (Busato et al., 2012a): PCMs in solar heating/cooling plants perform better than sensible heat storage only during periods when the mean temperature of the storage is around the melting temperature of the selected PCM. The

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economic analysis reveals that the greater investment cost of PCM technology is justified in a solar cooling plant only in the case of a higher natural gas tariff or a well lower PCM cost. During 2013 this research will

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be further developed: the hot side tank of the plant at ENEA Casaccia Research Centre will be replaced by a S44 filled tank (Busato et al., 2012b); experimental data will be collected and energy comparison will be further implemented.

ACKNOWLEDGMENTS

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This work was supported by a Research Program between University of Padova and ENEA “Progettazione di un serbatoio di accumulo a cambiamento di fase” (“Designing of a PCM storage system in solar cooling”). Authors thank ENEA (in particular Dr. Andrea Calabrese) to have permitted the publication of some of the

REFERENCES

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research results. Authors thank also Dr. Matteo Menin for the support in developing the simulation models.

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AA.VV., TEST REFERENCE YEAR TRY, 1985. Data Sets for Computer Simulations of Solar Energy Systems and Energy Consumption in Buildings. Commission of the European Communities. Directorate General XII for Science, Research and Development. Abhat, A. et al., 1981. Development of a modular heat exchanger with an integrated latent heat storage. Report no. BMFT FBT 81-050. Germany Ministry of Science and Technology Bonn. Agyenim, F., Hewitt, N., Eames, P., Smyth, M., 2010. A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS). Renew. Sustain. En. Rev. 14, 615-628.

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Baetens, R., Jelle, B.P., Gustavsen, A., 2010. Phase change materials for building applications: A state-ofthe-art review. En Build 42, 1361-1368. Bony, J., Citherlet, S., 2007. Numerical model and experimental validation of heat storage with phase change

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materials. En. Build. 39, 1065–1072. Bruno, F., 2005. Using Phase Change Materials (PCMs) for Space Heating and Cooling in Buildings. EcoLibrium, J. Aust. Inst. Refrig., Air Cond. Heat. 4(2), 26-31.

Buddhi, D., Sawhney, R.L., 1994. In: Proceedings on thermal energy storage and energy conversion.

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Busato, F., Lazzarin, R., Noro, M., 2012a. Energetic optimization of a solar cooling plant with PCM heat storage. Proceedings “10th IIR Gustav Lorentzen Conference”, Delft, ISBN 978-2-913149-90-8.

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Busato, F., Lazzarin, R., Noro, M., 2012b. Progettazione di un serbatoio di accumulo a cambiamento di fase (Designing of a PCM storage system in solar cooling) (in Italian). Report RdS/2012/125, ENEA. Cabeza, L.F., Castell, A., Barreneche, C., De Gracia, A., Fernández, A.I., 2011. Materials used as PCM in thermal energy storage in buildings: A review. Renew. Sustain. En. Rev. 15(3), 1675-1695. Calabrese, A., Fanchiotti, A., 2012. Messa in funzione, analisi sperimentale e caratterizzazione

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dell’innovativo impianto di solar heating and cooling realizzato a servizio dell’Edificio F92 (Operation, experimental analysis and characterization of the innovative solar heating and cooling plant at F92 building) (in Italian). Report RdS/2012/122, ENEA.

Chidambaram, L.A., Ramana, A.S., Kamaraj, G., Velraj, R., 2011. Review of solar cooling methods and

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thermal storage options. Renew. Sustain. En. Rev. 15, 3220-3228. ESTIF, 2012. Solar Thermal Markets in Europe. Trends and Market Statistics 2011.

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Fan, L., Khodadadi, J.M., 2011. Thermal conductivity enhancement of phase change materials for thermal energy storage: A review. Renew. Sustain. En. Rev. 15, 24-46. Farid, M.M., Khudhair, A.M., Razak, S.A., Al-Hallaj, S., 2004. A review on phase change energy storage: materials and applications. En. Convers. Manag. 45, 1597-1615. Felix Regin, A., Solanki, S.C., Saini, J.S., 2008. Heat transfer characteristics of thermal energy storage system using PCM capsules: A review. Renew. Sustain. En. Rev. 12, 2438–2458. He, B., Setterwall, F., 2002. Technical grade paraffin waxes as phase change materials for cool thermal storage and cool storage systems capital cost estimation. En. Convers. Manag. 43, 1709–1723.

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Himran, S., Suwono, A., Mansoori, G.A., 1994. Characterization of Alkanes and Paraffin Waxes for Application as Phase Change Energy Storage Medium. En. Sources 16(1), 117-128. Kenisarin, M., Mahkamov, K., 2007. Solar energy storage using phase change materials. Renew. Sustain. En.

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Rev. 11(9), 1913-1965. Khudhair, A.M., Farid, M.M., 2004. A review on energy conservation in building applications with thermal storage by latent heat using phase change materials. En. Convers. Manag. 45, 263–275.

Lazzarin, R., 2007. Solar cooling plants: how to arrange solar collectors, absorption chillers and the load. Int.

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J. Low Carb. Technol. 2(4), 376-390. doi:10.1093/ijlct/2.4.376

Medrano, M., Yilmaz, M.O., Nogués, M., Martorell, I., Roca, J., Cabeza, L.F., 2009. Experimental

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evaluation of commercial heat exchangers for use as PCM thermal storage systems. Appl. En. 86, 2047– 2055.

Oró, E., Gil, A., Miró, L., Peiró, G., Alvarez, S., Cabeza, L.F., 2012. Thermal energy storage implementation using phase change materials for solar cooling and refrigeration applications. En. Proc. 30, 947 – 956.

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Osterman, E., Tyagi, V.V., Butala, V., Rahim, N.A., Stritih, U., 2012. Review of PCM based cooling

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technologies for buildings. En. Build. 49, 37-49.

Paris, J., Falardeau, M., Villeneuve, C., 1993. Thermal storage by latent heat: a viable option for energy

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conservation in buildings. En. Sources 15, 85–93. Sharma, A., Tyagi, V.V., Chen, C.R., Buddhi, D., 2009. Review on thermal energy storage with phase change materials and applications. Renew. Sustain. En. Rev. 13, 318–345. Streicher, W. (edited by), 2008. Report C5 of Subtask C of IEA Solar Heating and Cooling programme Task 32 - “Advanced storage concepts for solar and low energy buildings”. Vakilaltojjar, S.M., 2000. Phase change thermal storage system for space heating and cooling. Ph.D. Thesis, School AME, University of South Australia.

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HIGHLIGHTS

A description of the features of thermal storage in solar cooling plants is given Characteristics of phase change materials (PCM) in solar energy storage are discussed

The plant has been simulated in Trnsys with sensible and PCM storages

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Results of the energy and economic analysis are discussed

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A real solar heating and cooling plant and its operation are described

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FIG.S CAPTIONS Fig. 1 - Different types of thermal storage of solar energy (adapted by Sharma et al., 2009).

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Fig. 2 - Classification of PCMs (adapted by Sharma et al., 2009). Fig. 3 - A simplified functional diagram of the HVAC solar cooling/heating plant at F92 building.

Fig. 4 – Energy balances in solar heating (a) and cooling (b) operations of the plant (see also Fig. 3).

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Fig. 5 – Enthalpy-temperature curves of the three PCMs (S7, S44, S89) used in the simulation.

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Fig. 6 – Main energy quantities obtained by simulation (for every month of HVAC plant operation) and by energy meters (for the periods 9th February till 15th April and 1st June – 15th September 2012): heating season (left) and cooling season (right). Fig. 7 - Comparison between the four types of storage (sensible, warm (PCM S44), hot (PCM S89) and cold (PCM S7)) for three different tank capacity pairs (the first number refers to the hot side tank, the second one to the cold side tank, in liters) in terms of energy balance of the main quantities.

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Fig. 8 - Comparison between the four types of storage (sensible, warm (PCM S44), hot (PCM S89) and cold (PCM S7)) for three different tank capacity pairs (the first number refers to the hot side tank, the second one to the cold side tank, in liters) in terms of Primary Energy consumption (PE), Primary Energy Saving (PES) and solar ratio.

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Fig. 9 – Temperature inside the solar tank with S44 PCM for five week-days of November.

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Fig. 10 – Net Present Worth and Discounted PayBack period of the solar heating/cooling plant, varying the capacity and type (sensible and PCM) of storages, in comparison with the “traditional” plant.

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TABLES AND TABLES CAPTIONS Table 1 - Thermophysical properties of products by Rubitherm GmbH (Kenisarin and Mahkamov, 2007). Melting Heat of Sensible heat Heat point fusion solid/liquid conductivity -1 -1 -1 (°C) (kJ kg ) (kJ kg K ) (W m-1 K-1)

Density solid/liquid (kg l-1)

Recomm. Volume Heat storage operating expansion capacity temper. range (%) (kJ kg-1) (°C)

Price of PCM (€ ton-1)

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Trade mark

~8

~140

1.8/2.4

0.2

0.86 (-15 °C)/0.77 (15 °C)

10

(-3)–+12

174

2900–3500

~28 ~45 ~43 ~52 ~55 ~59 ~64 ~81 ~90 ~99 ~81

~146 ~125 ~150 ~138 ~148 ~154 ~154 ~140 ~163 ~137 ~119

1.8/2.4 1.8/2.4 1.8/2.4 1.8/2.4 1.8/2.4 1.8/2.4 1.8/2.4 1.8/2.4 1.8/2.4 1.8/2.4 2.0

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

0.87 (15 °C)/0.75 (70 °C) 0.88 (15 °C)/0.76 (70 °C) 0.88 (15 °C)/0.76 (70 °C) 0.90 (15 °C)/0.76 (70 °C) 0.90 (15 °C)/0.77 (70 °C) 0.90 (15 °C)/0.76 (70 °C) 0.91 (15 °C)/0.79 (70 °C) 0.92 (15 °C) /0.77 (100 °C) 0.93 (15 °C)/0.77 (100 °C) 0.94 (15 °C)/0.77 (130 °C) 0.90 (20 °C)

10 10 10 10 10 10 10 10 10 10 8

19–35 35–50 36–51 40–55 46–61 48–62 56–71 71–86 82–97 91–106 71–86

179 155 174 167 179 181 173 175 194 168 149

2900–3500 2900–3500 2900–3500 2900–3500 2900–3500 2900–3500 2900–3500 2900–3500 2900–3500 2900–3500 6000

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RT 6 RT 27 RT 41 RT 42 RT 52 RT 54 RT 58 RT 65 RT 80 RT 90 RT 100 PK 80 A6

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Table 2 – Main characteristics of the equipment of the HVAC solar cooling/heating plant.

Li-Br absorption chiller

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Natural gas boiler

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Evaporative cooling tower

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Dry-cooler

3.75 m2 56 m2 0.718 0.974 W m-2 K-1 0.005 W m-2 K-2

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Solar field

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Evacuated tubes (mod. SKY21 CPC58 Kloben) Collector gross area Solar field gross area Efficiency: η0 (zero-loss efficiency) a1 (1st order heat-loss coeffic.) a2 (2nd order heat-loss coeffic.) Mod. YAZAKI_WFC-SC5 Rated cooling power Rated input thermal power T Heat Medium Inlet T Heat Medium Outlet T Chilled Water Inlet T Chilled Water Inlet T Cooling Water Inlet T Cooling Water Outlet Rated thermal power Rated efficiency Efficiency @ 30% nom. capac. Mod. THERMAC mod. 4 TE– 15 Rated cooling power @ Twb=25.6 °C; TH2O,in=35 °C; TH2O,out=30 °C Rated air flow rate Rated water flow rate Mod. ALFA LAVAL mod. DGS401AS BO Rated power Rated electrical power Rated air flow rate Volume

18 kW 25 kW 88 °C 83 °C 12.5 °C 7 °C 31 °C 35 °C 43.9 kW 88.4% 90.1%

43 kW 7500 m3 h-1 7400 l h-1

36 kW 0.2 kW 3200 m3 h-1 30 dm3

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Table 3 – Main thermo-physical characteristics of the PCM considered. Phase Change Temperature (ºC)

Density (kg m-3)

Latent Heat Capacity (kJ kg-1)

Volumetric Heat Capacity (MJ m-3)

Specific Heat Capacity (kJ kg-1 K-1)

Thermal Conductivity (W m-1 K-1)

S89 S44

89 44

1550 1584

151 100

234 158

2.480 1.610

0.670 0.430

S7

7

1700

150

255

1.85

0.4

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PCM type

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Table 4 – Input data of the economic analysis. Investment cost is considered to cover tanks (whose costs are reported in Table 5). The percentage of difference of the investment cost of the “traditional” plant is respect to the solar plant to satisfy the same heating and cooling loads. % difference cost 30%

Boiler plant cost (€) 19740

NG cost (€ Nm-3) 0.8638

Interest rate 3.0%

Period (y) 15

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Solar plant cost (€) 30000

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Table 5 – Input data of the economic analysis: investment cost in PCMs (cost of the PCM tube: 12.5 GBP; exchange rate 1 GBP = 1.1613 EUR in February 2013; administrative and supply costs by (Busato et al., 2012b)).

50 100 160 50 100 160 30 50 100

625 1250 2000 625 1250 2000 375 625 1250

495

895

Total cost (GBP) 2015 2640 3390 2015 2640 3390 1765 2015 2640

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Total cost (EUR) 2340 3066 3937 2340 3066 3937 2050 2340 3066

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Supply cost (GBP)

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Administrative cost (GBP)

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S7

Cost of tubes (GBP)

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S89

No. of tubes

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S44

Hot sidecold side tank capac. (l) 1000-500 2000-1000 3000-2000 1000-500 2000-1000 3000-2000 1000-500 2000-1000 3000-2000

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Table 6 – Input data of the economic analysis. Tanks costs in function of capacity, primary energy savings and economic savings of the solar heating/cooling plant with respect to the “traditional” one. Prim. En. Saved (Nm3 y-1)

Prim. En. Saved (€ y-1)

1686 1959 2034 2231

1457 1693 1757 1927

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Cool tank Tank cost Tank capacity (l) (€) 500 1000 1000 1500 1000 1500 2000 2100

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Hot tank Tank capacity Tank cost (l) (€) 1000 1500 1500 1800 2000 2100 3000 2600

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Thermal Energy Storage

Chemical

Thermal Chemical Pipe Line Sensible Heat

Latent Heat

Heat of Reaction

Solids

Solid-Liquid

Liquid-Gaseous

Solid-Solid

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Liquids

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Heat Pump

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Figure 1

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Thermal

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Phase Change Materials Paraffin Compounds

Organic

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Fatty acid Non-Paraffin Compounds

Other non-paraffin Salt Hydrate

Metallics

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Organic-Organic

Inorganic-Inorganic

Eutectic

Inorganic-Organic

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Figure 2

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Inorganic

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a)

SC01_Loss

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Diss

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Tank_to_load

FE07

b)

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SC01_Loss

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Loss_coll

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Figure 4

G_coll: solar energy on the plane of thermal collectors; Loss_coll: energy lost by solar collectors; Diss: energy dissipated by dry-cooler; FE01: useful thermal energy collected by solar field; SC01_Loss: energy lost by SC01 heat exchanger; FE02: useful solar thermal energy to solar tank; Tank_to_load: useful thermal energy supplied by solar tank (to the building in heating mode, to the generator of the absorption chiller in cooling mode); FE03: thermal energy integrated by boiler (direct to the building in heating mode, to the generator of the absorption chiller in cooling mode); FE07: thermal/cooling building needs; FE06: cooling energy produce by chiller; FE05: thermal energy supplied to the generator of the absorption chiller in cooling mode; FE04: thermal energy dissipated by cooling tower; Cool tank: cooling energy stored; Cool_tank_Loss: cooling energy lost by cold tank (thermal gain).

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Loss_coll

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FE03

+ FE04

FE06

Cool tank

Cool_tank_Loss

FE07

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