Retrofitting of the HVAC plant of an elderly people residential care home

About 14% of European population is over 65 years of age, and it is expected that this number will double by 2050. Care homes operate 24 hours a day, 365 days a year, with full occupancy. Some of the best available technologies were compared by an energy and economic analysis for a residential care home for elderly people in Vicenza (North East of Italy).

Keywords: Energy efficiency; HVAC plant; cogeneration; trigeneration; photovoltaic

Introduction

Among the public (but not only) buildings, residential care homes for elderly people are ones of the potentially most important because of the increasing European population aging. Nowadays about 14% of European population is over 65 years of age, and it is expected that this number will double by 2050 [1]. These figures in Italy assume dramatic values, as actually we have 21.4% of population over 65 (13 million) and we will have 33.6% (18.7 million) by 2050 [2][3]. Nowadays about 1.5 million elderly live in more than 24,000 care homes in Europe (300,000 elderly live in just less than 6,000 in Italy). These buildings operate 24 hours a day, 365 days a year, with full occupancy. For these reasons reducing energy consumption in residential care homes is important.

In this context, this paper focuses on the study of the energy performance of a quite new residential care home for elderly people in Vicenza (North East of Italy). A preliminary energy audit was carried out in order to obtain appropriate information about energy consumption of the building. Moving from the request of the managers responsible for running the care home, a Trnsys simulation model of the building and heating/cooling plant system was developed aiming at testing different solutions to retrofit the heating/cooling plant.

Building description and energy loads calculation

The considered building is a 29,889 m³ residential care home for elderly people built during the 2002–2004 period and located in Vicenza (North Italy). The main data of the building are reported in Table 1. The model of this three floors building (about 2,800 m² each) was implemented using “Trnsys3d for SketchUp” (Figure 1) and “TRNbuild”.

Table 1. Main data of the building.

An annual simulation of both heating and cooling loads was run with one-hour time step: annual heating and cooling energy use of the building were estimated to be 622,000 kWh and 529,000 kWh, with peak loads of 610 kW and 707 kW respectively. Concerning electric loads, there was availability of disaggregated data (on a quarter hourly basis) from local electric distributor for the years 2013 and 2014. Nevertheless, such data was subject to the different environment conditions (e.g. summer 2014 was extremely colder than usual in Italy) and building operating conditions. So, with some approximation based on information found by talks with technical personnel of the home, an hourly electric load profile was built taking into consideration monthly electric consumptions (deduced by electric bills), the actual installed electric power (400 kW) and the real data.

Heating/cooling plant description and primary energy consumption calculation

The existing heating, ventilation and cooling plant is mixed air/water; it is set up by:

·         heating and cooling plant (vapour compression electric chiller and natural gas boilers);

·         ventilation and air conditioning plant (many air handling units (AHU) to control relative humidity and for the necessary air changing inside the building; main units are set up by a rotary heat exchanger, pre-heating, cooling and dehumidification and post-heating sections). The common sites of the building (corridors, halls) are served by fan-coils and ventilation air, rooms and bathrooms are served by radiators and ventilation air, service and technical rooms by small AHUs or air heaters;

·         air extraction plant (for service rooms).

·         The object of the work is the energy and economic analysis of the retrofitting of the heating and cooling plant only. The main components are:

·         one air/water vapour compression electric chiller (2 circuits – 4 oil free centrifugal compressors per circuit) which provides cold water for fan-coils and cold coils in AHUs. The nominal cooling power is 895 kW, 270 kW is the nominal electric power consumption, EER = 3.31. These performances are labelled for summer external air 35°C and evaporator input/output 12/7°C;

·         two natural gas boilers with two-steps burners (nominal useful power 670 kW, minimum useful power 425 kW each) provide the thermal energy for heating, domestic hot water and pre and post-heating in AHUs.

The chiller and the boilers supply three hydraulic circuits. The “hot collector” supplies hot water to the radiators, air heaters, pre and post heating coils in the AHUs. It supplies also the domestic hot water plate heat exchanger that loads a 5,000 l tank. The “hot/cold collector” supplies hot (during heating season) or cold (during cooling season) water to the fan-coils. The “cold collector” supplies cold water to the cold coils of the AHUs (obviously during cooling season only). Reference [4] reports the temperature set points of the different energy uses and the schedules of the main equipment.

The first step was the simulation by Trnsys model of the just described existing plant (“As Is” solution). The validation of the model was carried out by analyzing flow rates and temperatures of the different circuits and by comparing simulated and real energy consumptions of the heating/cooling plant system. Figure 2 reports the annual profiles of monthly electrical energy (EE) and natural gas (NG) consumptions, both simulated and real (referring to 2012, 2103 and 2014 available energy bills). Considering the variability of the environment and building operating conditions, the concordance between simulated and real data is quite good (the slight overestimate of electrical energy consumption during mid seasons is due to the wider cooling period in the simulations with respect to the reality).

Figure 2. Annual profile of monthly electrical energy and NG consumptions

Alternatives for the heating/cooling plant energy retrofitting

Different efficient technologies among the most common known were taken into consideration for the energy improvement of the heating/cooling plant, chosen referring to previous economic estimates in possession of the managers responsible for running the care home. Main technical data of all the equipment considered in the present analysis are reported in [4]. For the sake of brevity, extremely concise information is here reported:

·         photovoltaics, PV: different kinds of mono and poli-crystalline modules were considered, varying the peak power in the 230–315 Wp range (14-16.9% nominal efficiency), considering a fixed available area (two terraces of 108 m² each on the roof and three parking shelters for 1,022 m²). In one alternative it was supposed to be able to install more PV modules on the roof (available area 350+350 m²). A penalization factor of −0.60% per year was considered in order to take into account the annual decrease of useful power. All the other parameters and input in Trnsys model were set in order to suitably simulate the PV plant;

·         combined heat and power, CHP (cogeneration): natural gas fueled internal combustion engines were considered with two different electrical nominal power (103 and 199 kW) and two different control strategies (electric load following and electric load following but during the 6.00 am-9.00 pm period only). A smaller size one was further considered (60 kW nominal power, electric - load following);

·         combined cooling, heat and power, CCHP (trigeneration): in order to extend the operation hours of the cogeneration plants, three of the previous cases were implemented considering to use the heat produced by the engine also during cooling season by coupling suitable sized lithium bromide-water single effect absorption chiller;

·         heat pumps: different sizes of vapour compression air/water heat pumps with both electric motor (EHP) and natural gas engine (GEHP) were considered. For the characterization of their performances the Authors referred to the UNI/TS 11300-4 and UNI EN 15316-4-2 method [5][6][7].

It was assumed to maintain the two actually present boilers with the double scope of both thermal integration and backup. Finally, primary energy consumptions were calculated taking into account thermal and electrical nominal efficiencies (on Lower Heating Value) for the cogenerators and nominal performances (COP) for the heat pumps, and considering their variations with the partial load (data derived by motors suppliers for the cogenerators – data derived implementing the UNI/TS 11300-4 and UNI EN 15316-4-2 calculation methods for quantifying energy loads and efficiencies of electrical HP-based heating plants).

Table 1. Costs of the different heating/cooling plant retrofitting solutions (*Refer to the HP nominal thermal power at the condenser at A7/W45 conditions) (**Electric motor efficiency=0.95 – Internal combustion engine efficiency=0.3 [7]).

Investment costs were determined by purchase, installation and first set up, besides costs of the existing plant adaptation [7][8][9][10] (Table 1). The same table reports on the ordinary maintenance costs of the different solutions, while such costs for the existing plant are considered to be 5,000 € year-1. Reference [4] reports on the costs of energy: natural gas and electricity purchased by the local distributors.

Concerning the management of the electricity produced by the cogenerators and photovoltaic plant besides own consumption, it was considered to be sold to the GSE (Energy Services Manager) at a constant price (fixed in 5.5 c€ kWh-1). For the alternatives here considered, the incomes from the energy efficiency certificates resulted negligible because they did not, or only slightly, satisfy the minimum primary energy saving index provided by the 2004/8/EC directive [11]. Finally, for the primary energy calculations we considered the conversion factors reported in [12][13], i.e. 1 kWhpr kWhth-1 and 2.17 kWhpr kWhel-1 respectively for natural gas and electricity.

Results and discussion

The solutions that maximize NG consumptions (even if minimizing the boilers consumption) are the ones with the installation of the 199 kWel nominal power cogenerators, while the solutions minimizing the consumption in absolute terms concern installing the biggest sized electric heat pumps (“EHP_311 kWt” and “EHP_424 kWt”). The balance, in terms of electrical energy, takes into account the “self-produced” and the “sold to grid” electricity: the solutions that minimize the purchase from the grid (by more than 20 times with respect to the “As Is” plant) are the ones that maximize the self-production (“2,  199 kW – el. following” with and without trigeneration).

A more comprehensive comparison, carried out in terms of primary energy (PE), is reported in specific terms (per square meter of building surface) and in absolute terms as well in Table 2. Self-produced electrical energy accounts for a negative value in terms of consumption, so the best solutions foresee the installation of 199 kWel cogenerator (coupling the absorption chiller allows a very slight improvement). These solutions allow to reduce by 63% the PE consumption. Anyway, if one looks at the benchmark for Italy suggested by [1] (234 kWh m-² year-1), all the CHP and CCHP alternatives and two PV solutions are performant. On the other hand, heat pumps, both electric and natural gas driven, are not advantageous at all and, in some cases, they lead to an increase of the PE consumption. This is due to the penalized operation of air/water heat pumps in the Vicenza winter climate with frequent defrosting necessity ([9][10][14]) and especially due to the high temperature of water to be produced at the condenser (70°C) serving the radiators and the pre and post heating coils of AHUs.

Table 2. Annual Primary Energy consumption of the different solutions, both absolute (MWh) and specific (kWh m-²)

The interest rate, the inflation rate and the period of the economic analysis were fixed at 4%, 1.8% and 20 years respectively. From the economic point of view, the comparison between the different alternatives did not consider the investment cost of substituting the existing plant, as it was considered to be a sunk cost (Table 1). The best solutions in terms of trade-off between maximum differential (investment alternative versus “As Is” solution) net present worth (NPW) and minimum discounted pay-back period (DPB) are the CHP/CCHP ones with installed electrical power around or less than 100 kWel. In these cases, NPW is around 630-670 k€ and DPB is between 3 and 4 years (Figure 3); these are very interesting results, also considering that we are talking about the plant of a public building stock (so payback periods can be quite longer than in industry sector). These are also the solutions that allow the minimization of thermal energy dissipation by the cogenerator. From this point of view, it is interesting to observe that a further increase of the economic viability of such alternatives would be obtained by operating the cogenerator with the thermal load following logic.

Concerning the “2, 199 kW” solution it is worth to stress that coupling the absorption chiller can improve the economic viability, but even more advantageous is the operation during the day only (respectively NPW increases from 195 to 332 to 423 k€ and DPB decreases from 10.9 to 9.6 to 7.1 years). Photovoltaics is also interesting, even if it allows smaller NPWs (200-300 k€) and longer DPBs (9-11 years). Should be possible to use more surface on the roofs (“1, Solon Black 220/16 mono (265 Wp)__Plus” solution) the economic viability would increase (NPW=440 k€, DPB=8.6 years). It is interesting to observe that heat pump solutions are not advantageous at all as they present a greater than “As Is” solution annual cash flow for the reasons explained before.

In reference [4] these conclusions are completed by a sensitivity analysis in order to compensate for the uncertainty of some of the parameters here considered, such as natural gas and electrical energy costs.

Figure 3. Discounted differential (between the alternatives and the “As Is” solution) cash flows of the different solutions. NPWs can be read at the end of the period of analysis (20 years), DPBs by the interceptions of the curves with the x-axis.

Conclusions

Many energy efficiency interventions can be thought to be implemented in residential care homes for elderly people as the greatest part of them are old buildings, i.e. built before regulations on buildings energy performance and economic incentives. In this sense, interventions on the building (e.g. retrofitting of the opaque and transparent surfaces by thermal insulation and windows substituting) are the first ones that should be implemented; installing a solar thermal plant and substituting the old lighting appliances by more efficient ones should be the second ones. In more recent (and so more energy performant) buildings some retrofitting interventions in heating/cooling plant can be analyze. Photovoltaics, cogeneration, trigeneration, electric and gas engine heat pumps were considered in this study and the energy and economic viability were evaluated. Cogeneration with small size engines (with respect to the installed electric power by local distributor) result the most advantageous solutions, whereas coupling a single-effect absorption chiller do not significantly improve the advantage. Photovoltaics as well allows an interesting energy saving with respect to the existing plant, even if with longer payback periods. Air/water heat pumps (the most economic and widespread diffused ones) are not advantageous at all in this case because of the high temperature at condenser and because of the cold and humid winter climate typical of the Po Valley (that implies frequent defrosting of the evaporator coil). The main conclusions of this study will be delivered to the managers responsible for running the residential care home in order to make energy efficient informed decisions.

References

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[3]       ISTAT (Italian national statistics institute), 2006. Previsioni demografiche nazionali (National demographic forecasts, in Italian).

[4]       Noro Marco, Bagarella Giacomo, 2015. Energy and economic analysis of different solutions for the retrofitting of the heating and cooling plant for a residential care home for elderly people, Proceedings “14th International Conference on Sustainable Energy Technologies (SET-2015)”, Nottingham.

[5]       UNI/TS 11300-4:2012, Prestazioni energetiche degli edifici - Parte 4: Utilizzo di energie rinnovabili e di altri metodi di generazione per la climatizzazione invernale e per la produzione di acqua calda sanitaria (Buildings energy performances – Part 4: Use of renewable energies and other methods for heating and sanitary hot water production, in Italian).

[6]       UNI EN 15316-4-2:2008, Impianti di riscaldamento degli edifici - Metodo per il calcolo dei requisiti energetici e dei rendimenti dell'impianto - Parte 4-2: Sistemi di generazione per il riscaldamento degli ambienti, pompe di calore (Heating plants in buildings – Method for the calculation of energy requests and efficiency – Part 4-2: Heating generation systems for buildings heating, heat pumps, in Italian).

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[11]    Directive 2004/8/EC of the European Parliament and of the Council on the promotion of cogeneration.

[12]    MICA, 1992. Circolare del Ministero dell’Industria, del Commercio e dell’Artigianato del 2 marzo 1992, n. 219/F (Document of the Commerce and Industry Ministry).

[13]    AEEG, 2008. Delibera EEN 3/08, Aggiornamento del fattore di conversione dei kWh in tonnellate equivalenti di petrolio connesso al meccanismo dei titoli di efficienza energetica (Updating of the kWh/toe conversion factor related to the energy efficiency certificates, in Italian).

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