field monitoring in air conditioning systems: italian …mdepaepe/commisioning_auditing... ·...

Field monitoring of air conditioning systems in the tertiary sector: experiences in Italy MARCO MASOERO, Professor, Dipartimento di Energetica, Politecnico di Torino, Italy. [email protected] CHIARA SILVI, Research Scientist and Lecturer, Dipartimento di Energetica, Politecnico di Torino, Italy. [email protected] ABSTRACT This work was carried out within the AUDITAC project supporting the implementation of Article 9 of the EPBD (Inspection of air-conditioning systems of an effective rated output of more than 12 kW). One of the main goals of the project was to collect and organise into a data base a set of case studies, documenting real examples of audits leading to successful energy-saving actions. The data base should include examples from all the participating countries (Austria, Belgium, France, Italy, Portugal, Slovenia, and the United Kingdom) in order to cover as much as possible the different range of climatic conditions, building type and size, and AC system characteristics. In this article the most representative Italian Case Studies are presented. One of the main difficulties encountered in collecting the case studies was the lack of information on actual AC energy use. This is generally due to the fact that electricity consumption for refrigeration equipment, fans, pumps, etc. is centrally metered at the grid interface, and that disaggregation of individual users and separation from non-AC related ones (lighting, equipment, etc.) is often difficult; furthermore, an accepted consensus does not exist on the methods for correlating AC energy use and climate. To gain a better understanding on this topic, specific monitoring campaigns were set up for: (1) the AC system of a surgery / nursing department of a small hospital ; (2) the refrigeration equipment of a larger hospital complex; (3) a water-to-water heat pump system used for heating and cooling a small public building. The main findings of the three case studies are presented and discussed in this paper. 1. INTRODUCTION

The AUDITAC project (Bory and Adnot, 2006) - funded by the EC DG-TREN within the Energy Intelligent Europe (EIE) program for the 2005-2006 biennium - has developed a set of materials and procedures applicable to the Energy Auditing of existing air conditioning (AC) systems. One of the main goals of the project was to collect and organise into a data base a set of case studies, documenting real examples of audits leading to successful energy-saving actions. The data base should include examples from all the participating countries (Austria, Belgium, France, Italy, Portugal, Slovenia, and the United Kingdom) in order to cover as much as possible the different ranges of climatic conditions, building type and size, and AC system characteristics.

As soon as the search for case studies started, it clearly appeared to most participants that the available information on such audits was extremely limited, particularly as far as the actual energy consumption data were concerned. Several reasons lay behind this lack of information, namely:

• The main energy input for AC systems is the electricity used by the motors that drive refrigerating compressors, fans, and pumps; generally, electrical energy is centrally metered at the grid interface (main delivery board) without separating the individual users (i.e., lighting, appliances, AC, etc.).

• Most energy service contracts (particularly in commercial buildings) actually include AC systems. However, while for space heating energy metering is normally required as a means for determining the cost of the service, electricity bills are generally paid directly by the building owner / tenant; the contract therefore includes the AC system operation and maintenance costs only. Consequently, no real reasons exist for implementing a costly and relatively complex procedure of gathering disaggregated electricity use data.

Furthermore, most AC system retrofits carried out in the past (at least in Italy) were determined by reasons different from energy conservation, namely:

Field monitoring of air conditioning systems in the tertiary sector: experiences in Italy

• Improving the comfort condition in work spaces; • Solving IAQ problems, or complying with compulsory regulations on air changes (e.g. in hospitals); • Replacing room air conditioners with a central HVAC system to overcome maintenance problems and

to avoid excessive differences in indoor environmental conditions. A final problem that was encountered refers to the analysis of energy data. While for space heating the

Heating Degree-Day (HDD) method has been firmly established since decades as a simple and reliable means for correlating energy consumption and local climate, several approaches to summer space conditioning have been proposed, but none of them has reached so far a generalised consensus in the scientific and professional community.

Four case studies have been collected within AUDITAC by the Italian team. Two of them concern hospitals (Balducci and Cagliero, 2006) – one focused on air conditioning of a surgery / nursing department, the other on retrofits of refrigeration equipment; one case study focused on monitoring the performance of a water-to-water heat pump system (Cantarella and Dominguez, 2006); the last one on a historic building that was refurbished to accommodate office spaces. The main findings of the first three case studies are presented in this paper. 2. MONITORING THE AC SYSTEM OF A SURGERY / NURSING DEPARTMENT

This case study concerns the AC system serving the surgery / nursing department of a small hospital (158 beds) situated in the north-eastern Italy. The study was carried out in cooperation with the ESCO responsible of managing the energy systems of the hospital. The building and AC system under investigation are currently undergoing a complete renovation: the floor hosting the surgery / nursing department has been completed and a totally new AC system has been operating since the summer of 2005. Relevant data on the building and AC system are summarized in Table 1. Table 1 – Building and AC system characteristics Building construction Concrete framed with masonry walls Floor area of the department 350 m2 Air Conditioning system All-air (100% outdoor air) with HEPA filters on room terminals Air Handling Units (AHUs) Two identical AHUs (for each of the surgery / nursing areas) Air humidification Electrical steam humidifiers (one for each AHU) Heat recovery Intermediate-fluid heat recovery deck

Supply air post-treatment Three re-heating / re-cooling decks for operating rooms no. 1 and 2 and for recovery area

Supply air flow rate 9700 m3/h (for each AHU) Extract air flow rate 8800 m3/h (for each AHU) Supply fan electric power 11 kW (for each AHU) Extract fan electric power 4 kW (for each AHU)

Water chiller Roof -mounted, air-cooled water chiller with two hermetic compressors, refrigerant fluid R22

Refrigeration power 250 kW Electrical power input 45 + 45 kW (refrigerating compressors) + 5 kW (condenser cooling fans)

HVAC Plant Control: Continuous operation (24 hrs/day) for contamination control Each space of the departme has individual temperature control nt

Set Points (operating rooms) Adjustable in the 18-24 °C E 1°C range

The energy analysis has been focused on optimising the operation of the Air Handling Unit (AHU) of the department. The main operational parameters of the AHU (see Figure 1) were monitored in the April-October 2006 period. Four data acquisition modules were employed, each connected to four temperature sensors. Data were recorded at 15 min. intervals and periodically downloaded to a laptop PC. Electricity consumption of the heat recovery loop circulation pump was also measured. Data analysis was focused on: (1) ventilation air heat recovery; (2) free cooling with outdoor air.


• Outdoor air / Heat recovery inlet, air supply side (B11); • Return air / Heat recovery inlet, air discharge side (B12); • Heat recovery outlet, air supply side (B13); • Pre-cooling air outlet (B14); • Heat recovery outlet, air discharge side (B21); • Post-heating / Post-cooling air outlet operating room n° 1 (B22); • Post-heating / Post-cooling air outlet recovery room (B23); • Post-heating / Post-cooling air outlet operating room n° 2 (B24); • Pre-heating deck water supply (B31); • Pre-heating deck water return (B32); • Pre-cooling deck water supply (B33); • Pre-cooling deck water return (B34); • Heat recovery deck water supply (B41); • Heat recovery deck water return (B42); • Post-heating deck water supply operating room n° 1 (B43); • Post-heating deck water return operating room n° 1 (B44).

Figure 1 – Temperatures recorded for AHU monitoring

The average thermal effectiveness of the intermediate-fluid heat recovery system turned out to be on the order of 58% (based on measurements). For sake of comparison, an air-to-air heat exchanger (65% effectiveness), was also considered. A performance comparison for the period 23 June – 22 September 2006 (assuming that heat recovery is on when Tout – Tin > 2°C) yielded the results o Table 2. In terms of financial impact, this action lead to savings on the order of 300 € (500 € if air-to-air heat recovery had been adopted). Table 2. Heat recovery performance analysis (A = intermediate fluid heat recovery; B = air-to-air heat recovery)

Heat recovery type A B Δ (B–A) Recovered thermal energy (kWh) 2955 7819 4864 Chiller electrical energy savings (kWh) 1477 3910 2433 Heat recovery loop pump electrical consumption (kWh) 389 0 -389 Net electrical energy savings (kWh) 1088 3910 2822


Free cooling by direct supply of outdoor air (without mechanical cooling) is assumed feasible when Tout < 20°C. Seasonal expected energy savings are summarised in Table 3.

Table 3 – Seasonal expected energy savings with free cooling Free cooling YES NO Δ Δ(%)

Cooling energy (kWh) 48075 57079 9004 16% Chiller electrical energy (kWh) 24037 28539 4502 16%

The investigation suggested that a more extensive use of heat recovery and free cooling may lead to

significant energy savings. The following other Energy Conservation Opportunities (ECOs) were also proposed: • Installation of screens to protect the air-cooled condensers of the water chiller from direct solar

radiation; • Partial or total recovery of condenser heat for air re-heating; • Exclusion of the re-heating deck of operating room n° 2 (which is used for urgencies only), while

maintaining the prescribed air change; • Automatic closure of operating room doors to avoid energy losses due to treated air movement.

3. RETROFITTING THE REFRIGERATION EQUIPMENT OF A HOSPITALCOMPLEX

This case study is aimed at optimizing the operation of the refrigeration equipment present in a 300 beds hospital in North-West Italy. The hospital was built in the early 1960’s and, originally, was not equipped with a comprehensive centralised AC system. Decentralised AC systems (including, chiller, AHU and air / water networks) have subsequently been installed in selected areas. The study was carried out in cooperation with the ESCO which manages the AC system, in conjunction with planned renovation work foreseeing the installation of new chillers and the construction of a chilled water loop connecting the existing refrigeration units. Potential energy and cost savings for various options were examined, including: (1) replacement of existing chillers; (2) different strategies of chiller operation;(3) free cooling; (4) recovery of condensation heat for SHW production.

In the initial configuration, fifteen refrigeration units (identified as ECn = Existing Chiller n) were present in the hospital; the planned renovation work includes the installation of two new, identical refrigeration units (identified as NCn = New Chiller n), and the construction of a chilled water loop. The refrigerating power output of each of the new units (963 kW) is about equal to the sum of the outputs of existing chillers EC1, EC2 and EC3 (955 kW). The position of the existing chillers EC1, EC2 and EC3, of the new chillers NC1 and NC2, and of the chilled water loop is shown in the following Figure 2; chiller data are summarized in Table 4.

Figure 2 – Layout of the existing chillers (EC), new chillers (NC) and chilled water loop

Chillers EC1, EC2, EC3

Chillers NC1, NC2

Chilled water loop


Table 4. Characteristics of the existing chillers (EC) and new chillers (NC)

Unit no. Compressor electric power Refrigeratingpower

Water flow rate

Pump Electric power

Nominal COP

kW kW m3/h kW EC1 183 355 63 3.5 1.94 EC2 183 355 63 3.5 1.94 EC3 83 245 45 3 2.95 EC4 315 884 150 7,5 2,81 EC5 125 250 45 3 2.00 EC6 21 50 10 1.1 2.38 EC7 15.5 35 7 0.75 2.26 EC8 7 17.2 3 0.5 2.46 EC9 7 17.2 3 0.5 2.46

EC10 25 60 12 1.5 2.40 EC11 125 250 42 3 2.00 EC12 9.8 35 6 0.75 3.57 EC13 23 55 10 1.1 2.39 EC14 125 250 42 3 2.00 EC15 40 80 15 1.5 2.00 EC16 120 295 52 3.5 2.46 NC1 396 963 170 10 2,43 NC2 396 963 170 10 2,43

The existing and new chillers adopt different control strategies: two regulation steps for the existing chillers,

and nine regulation steps for new chillers. The chiller COP data used in the energy analysis are given in Table 5. Table 5 – Chiller COP data

Regulation steps

Regulation steps EC1

1 2 NC1

1 2 3 4 5 6 7 8 9 COP 2.70 1.94 COP 2.55 2.43 3.11 2.85 2.68 2.55 2.50 2.46 2.43

Lacking experimental data on cooling performance, the analysis was carried out mostly by simulation, using

the following approach: • Weather data: hourly data (temperature and relative humidity) for the average day of the warmest

months (April – September) measured at Milano-Linate airport were used. • Cooling load vs climate: the Humidex index (Masterton J.M., Richardson F.A., 1979), H, was used as

the single-value climate descriptor hourly values of H were calculated for the six months. It was assumed that cooling demand is a linear function of H, the peak cooling demand (equal to the chillers rated output) occurring for the maximum hourly value of H (H = 32.2°C at 16 hrs in August), and cooling demand becoming zero for H = 15°C. The cooling load fraction for each hour of the six months were then determined.

• Chiller performance: hourly COP values were calculated as a function of load fraction, using the performance data of section 5.

The following retrofit / system management options were analysed: • Replacing chillers EC1, EC2, EC3 with new chiller NC1; • Using both NC1 and NC2 at partial load; • Increasing the air-conditioned area; • Modifying the outdoor temperature at which chillers are shut off and free cooling is performed; • Recovering condensation heat for SHW.


2.1 Replacing chillers EC1, EC2, EC3 with new chiller NC1 New chiller NC1 has a rated refrigeration power output which is virtually equal to the total power output of

EC1 + EC2 + EC3. The analysis assessed the expected savings yielded by the replacement of the existing chillers with the new one (Figure 3). Expected seasonal electricity consumption reduction are on the order of 15730 kWh, yielding savings on the order of 1420 €/yr (i.e., 4% of present costs). Daily cost for electric energy and possible saving in the analyzed

period

-€ 10

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Obtained saving Old chillers New chillers

Figure 3 – Daily savings in electricity costs due to chiller replacement 2.2 Using both NC1 and NC2 at partial load

As an alternative option, both NC1 and NC2 operating at partial load could replace the existing chillers (Table 6). This strategy should achieve a higher overall chiller efficiency, while increasing the pumping energy (two pumps instead of two). Compared to the above option (NC1 only), further savings on the order of 1460 €/yr could be achieved. Table 6. Savings associated with using both new chillers at partial load

EC1+EC2+EC3 NC1 NC2+NC3 Chiller electrical consumption (kWh/yr) 391830 376100 339200 Pumps electrical consumption (kWh/yr) 21000 21000 41400 Total electrical consumption (kWh/yr) 412830 397100 380600 Total electricity costs (€/yr) 33250 31830 30370

2.3 Increasing the air-conditioned area

As a future option, the substitution of other existing groups with NC2 has been evaluated. Calculation was based on a peak load of 355 kW and an average COP for the replaced chillers. Expected seasonal savings are on the order of 1790 €/yr (i.e., 4% of present costs).


2.4 Modifying the outdoor temperature at which chillers are shut off and free cooling is performed Savings associated to a 1°C variation in the limit temperature at which the chillers are shut off and free

cooling is adopted (23°C vs 22°C) are approximately equal to 50,000 kWh/yr (with negligible differences between existing and new chillers), i.e. on the order of 12%. 2.5 Recovering condenser heat for SHW

As a base option, the new chillers are not equipped with condenser heat recovery system. The benefits associated with a partial recovery of condenser heat have been evaluated. (To achieve partial heat recovery, the condenser is subdivided into two sections: the water-cooled high-temperature section transfers the heat corresponding to the de-superheating phase of the process to the water, while the low-temperature air-cooled section rejects the heat of condensation to outdoor air.) By analysing the chiller thermodynamic cycle, the recovered heat was evaluated; it was further assumed that heat recovery is limited to the warmest period (six hours per day in July and August). The economic analysis based on Net Present Value (NPV) calculation yielded a 5.2 years payback time (Figure 4).

SHW production with condenser heat recovery Recovered condensation power 191,25 kW SHW temperature range (mains – delivery) 15 – 40 °C SHW demand per person 140 L/person-day Daily SHW energy demand per person 4.07 kWh/person-day Daily recovered heat of condensation 1147.5 kWh/day SHW volume produced with recovery 39474 L/day Number of people served 282

SHW production with natural gas boiler Boiler efficiency 0.85 Daily natural gas consumption 140.7 m3/day Daily cost 58,22 €

Costs analysis Seasonal savings (July and August) 3610 € Extra cost of the chiller 4500 € Cost of the storage tanks 10500 € Payback time 5.2 yrs

NPV

-€ 16.000

-€ 14.000

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€ 0

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0 1 2 3 4 5 6Years

Figure 4 – Economic analysis of partial recovery of condensation heat for SHW production


3. MONITORING THE SUMMER PERFORMANCE OF A WATER-TO-WATER HEAT PUMP

This case study was conducted in cooperation with the Brasimone Research Centre, established in the early 1960s by CNEN (National Committee for Nuclear Energy) – later to become ENEA (Italian National Agency for New Technologies, Energy and the Environment) - on the eastern shore of an artificial water basin, serving a nearby ENEL (National Electric Utility) hydroelectric power station. The facility is located in the Apennine mountain range, halfway between Bologna and Firenze, at 846 m above sea level. In the mid 1980s, a small building (1.800 m3) - including offices, an exhibition, area and a 100 seat conference room - was constructed on the side of the basin opposite the research centre. This initiative was jointly promoted by ENEA and ENEL to promote communication to the public on the activities being conducted by the two organisms in the Energy field (building views are shown in Figure 5 and the characteristics of the building and AC system are summarised in Table 7). In 2005, the HVAC system of the building has been completely renovated. The results of the system monitoring campaign, carried out in its first summer of operation (May – September 2006) are presented in this paper.

Figure 5 – ENEA-ENEL building views

The AC system is of the air-and-water type (primary air and two-pipe fan coils). Hot and chilled water is produced with a water-to-water reversible heat pump, using treated lake water as the heat source / sink. A newly installed BEMS allows continuous monitoring of the main performance parameters of the system. Eleven two-pipe fan coils units are installed in the conference room and exhibition area at the ground floor, and in the offices at the upper floor. Radiators, fed by a separate hot water circuit, are provided for the rest rooms. The AHU has a nominal supply air flow rate of 3200 m3/h (100% outdoor air with heat recovery) The AHU supplies fresh air to the conference room. Air is extracted partly from the conference room, partly from adjacent spaces.


The reversible water-to-water heat pump delivers a maximum thermal power of 60 kW (cooling @ 7-12°C) and 68 kW (heating @ 40-45°C). Condensation heat recovery in cooling mode is performed with a dedicated condenser. A scheme of the hydraulic circuits connecting the heat pump to the AHU and fan coils (primary circuit) and to the lake water (secondary circuit) is shown in Figure 6. The heat exchanger of the primary circuit is of the shell-and-tube type, and is immersed in an inertial storage of 200 litres. The heat exchanger on the secondary circuit is of the brazed plate type; the heat recovery condenser is also of the brazed plate type. A water-glycol solution is used in the secondary circuit to avoid the risk of freezing. The existing oil boiler was maintained for emergency use.

Figure 6 – Water-to-water Heat Pump hydraulic circuits

Table 7 – Building and AC system characteristics

Building construction: Concrete framed with masonry walls Conditioned floor area 300 m2 AHU Supply air flow rate 3200 m3/h AHU Extract air flow rate 2600 m3/h AHU Humidifier (steam) flow rate 10 kg/h Fan-coils (three independent circuits) Conference room, exhibition room, offices Radiators Rest rooms Heat Pump Cooling power 68 kW @ 7-12°C Heat Pump Heating power 60 kW @ 40-45°C Electrical power input 16.2 kW Compressor type Two hermetic scroll compressors Refrigerant fluid R407c Oil boiler 70 kW (existing)

The building is equipped with a BEMS operating at two hierarchical levels: a set of local control units

manage the individual HVAC components (terminals, AHU, heat pump), while a central PC performs the supervisory management. The central PC is capable of transmitting information to one or more external clients, similarly to a standard Internet Web server, the only requirement on the client side being the presence of an Internet browser and a password to access the website. The collected data (e.g, air / water temperatures, electrical energy consumption, malfunctioning alarms, operator intervention requests, etc.) are saved and can be downloaded by remote computers. The heat pump cooling / heating power output is regulated by on-off control of the two compressors: therefore two levels of power output are possible. The AHU is equipped with standard air temperature / humidity regulation. Room thermostats control fan-coil operation.

The main results of the monitoring campaign carried out in the summer of 2006 are summarized in the following charts and graphs (all data were obtained from the system BEMS and remotely downloaded on a PC):

SECONDARY CIRCUIT

AHU

FAN COIL

PRIMARY CIRCUIT HEAT PUMP

TO FROM LAKE LAKE


• The monthly average COP (Figure 7) was computed from the measured data of delivered cooling energy and compressor electrical consumption; the seasonal average COP turned out to be 3.9. Similarly, the thermal energy input obtained from the lake water was measured (Figure 8).

• A correlation analysis was performed to investigate the dependence of delivered cooling energy (AC system thermal load) on outdoor climate. The graphs of Figure 9 show the dependence of cooling energy on air temperature, specific humidity and enthalpy. The best correlation is obtained when air temperature is considered. This fact may be explained by considering that, during the period of investigation, the AHU fans were generally switched off (the conference room was mostly unoccupied): the AC cooling load was therefore primarily determined by solar and conduction gains, which are fairly well correlated with outdoor dry-bulb air temperature.

• Finally, the heat pump load factor was determined by analysing the compressors duty cycle (Figure 10). The capacity control is in fact on-off: therefore, the heat pump load factor can be determined by measuring the time fraction for which the each compressors are on.

0,00,51,01,52,02,53,03,54,04,5

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Figure 7 - Monthly average C.O.P. and outdoor temperature

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h ]

Electrical consumption [ kWh ] Lake's contribution [ kWh ]

Figure 8 -Compressor electrical consumption and thermal energy input from lake water


Delivered cooling energy and outdoor air enthalpy (daily values):

Delivered cooling energy and outdoor specific humidity (daily values):

Delivered cooling energy and outdoor specific humidity (daily values):

Delivered cooling energy and outdoor specific humidity(daily values)

Figure 9 - Cumulative frequency of heat pump utilisation factor

Supplied energy

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[ kJ

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[ kJ

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Figure 9 - Cooling energy vs. air temperature, air specific humidity and air enthalpy


Figure 10 – Heat Pump load factor

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4. CONCLUSIONS

The implementation of EPBD’s article 9 on the inspection of AC systems of effective output of more than 12 kW may offer a unique opportunity to promote effective energy savings policies in building air conditioning. However, in order to transform this opportunity into a real widespread market, several technical and institutional barriers still have to be overcome, as the activities carried out within the AUDITAC project have indicated:

• National legislations must provide clear guidance on AC inspection, in terms of timing, methodologies, reference standards, official inspecting bodies , etc.;

• Incentives should be adopted in order to promote energy service contracts including clauses that – similarly to what is already customary in space heating – remunerate electrical energy savings in summer air conditioning;

• Technical standards, accepted by the professional and scientific community, are needed both for the calculation of summer AC energy and for the evaluation of ECOs;

• Provisions for disaggregated electricity use metering should become customary in new installations and incentives for retrofitting the existing one should also be introduced.

To overcome these barriers, a concerted action will be necessary in the coming years involving, at the Community level, more EC funded research and CEN activities, and, at the National level, an effort to complete the implementation of the EPBD to include summer air conditioning. 5. ACKNOWLEDGEMENTS

This work is part of the AUDITAC project (Field Benchmarking and Market development for Audit methods in Air Conditioning) funded by the EC DG-TREN within the Energy Intelligent Europe program for the 24-month period January 2005 to December 2006. The authors wish to thank APS Sinergia S.p.a., SIRAM S.p.a. and ENEA-Brasimone for the assistance provided in this research. The assistance of the Master’s candidates S. Balducci, F. Cagliero, A. Cantarella, and D. Dominguez Michelangeli is gratefully acknowledged. REFERENCES

Balducci, S. and Cagliero, F. 2006. Analisi energetica degli impianti di climatizzazione estiva nelle strutture ospedaliere. Master’s Thesis in Mechanical Engineering, Politecnico di Torino.

Bory, D. and Adnot, J. 2006. Overall on Auditac Project: Field benchmarking and Market development for Audit methods in Air Conditioning project Audit Procedures. Proceedings AICARR Conference Milano 2006.

Cantarella, A. and Dominguez Michelangeli, D. 2006. Analisi sperimentale delle prestazioni energetiche e funzionali di un impianto a pompa di calore acqua-acqua. Master’s Thesis in Mechanical Engineering, Politecnico di Torino.

Masterton, J.M. and Richardson, F.A. 1979. Humidex, a method of quantifying human discomfort due to excessive heat and humidity, Report CLI 1-79, Environment Canada, Atmospheric Environment Service, Donsview, Ontario.

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