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Renewable and hydrogen energy integrated house

E. Bocci a,*, F. Zuccari b, A. Dell’Era b

aCIRPS, University of Rome ‘‘La Sapienza’’, Via Eudossiana 18, 00184 Rome, ItalybUniversity of Rome “G. Marconi”, Via Plinio 44, 00193 Rome, Italy

a r t i c l e i n f o

Article history:

Received 26 May 2010

Received in revised form

10 January 2011

Accepted 18 January 2011

Available online 26 February 2011

Keywords:

Energy efficiency

Renewable energy

Hydrogen

House

* Corresponding author.E-mail address: enrico.bocci@uniroma1.i

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.01.098

a b s t r a c t

The residential sector accounts for about a third of the total world energy consumption.

Energy efficiency, Renewable Energy Sources and Hydrogen can play an important role in

reducing the consumptions and the emissions and improving the energy security if inte-

grated (Efficiency, Res, Hydrogen) systems are developed and experimented. The paper

analyzes a real residential 100 square meters house, where energy efficiency measures and

RES technologies have been applied, sizing a hydrogen system (electrolyzer, metal hydrides

and fuel cell) for power backup, taking into consideration its dynamic behavior, experi-

mentally determined. The technologies used are already available in the market and,

except hydrogen technologies, sufficiently mature. Through energy efficiency technologies

(insulation, absorbers, etc), the maximum electrical and thermal power needed decreases

from 4.4 kWe to 1.7 kWe (annual consumption from 5000 kWh to 1200 kWh) and from

5.2 kWt to 1.6 kWt (annual consumption from 14,600 kWh to 4500 kWh) respectively. With

these reduced values it has been possible to supply the consumptions entirely by small

photovoltaic and solar thermal plants (less than 10 m2 each). The hydrogen backup even if

remains themost expensive (versus traditional batteries and gasoline generator), satisfying

all the electric needs for one day, increases the security and allows net metering. Moreover

the low-pressure hydrogen storage system through metal hydrides guarantees system

safety too. Finally the system modularity can also satisfy higher energy production.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction and the first sector in the industrialized countries like Europe,

To realize a sustainable energy system we must change the

whole energy chain: from the supply sector to the energy end-

use technologies. The crucialmission of energy research is the

promotion and implementation of methods, technologies and

processes to develop clean (no resources consumption and

zero pollutant emissions), efficient, appropriate (to different

local conditions), safe and convenient energy systems. This is

particularly true for the households and services sector, which

is the third energy sector in the world energy consumption [1]

t (E. Bocci).2011, Hydrogen Energy P

as showed in the Fig. 1 [2].

Moreover the diffused structure of the residential

consumption if, from one side, can be a difficult to change,

from other side, is a perfect structure to implement distrib-

uted energy generation systems sourced by local renewable

energy [3]. The development of integrated Energy Efficiency,

Renewable Energy Sources and Hydrogen systems can play an

important role in reduce the consumptions and the emissions

and improve the energy security if integrated systems are

developed and experimented [3].

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Fig. 1 e European energy consumption by sector [2].

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1.1. The case study

The paper considers like case study a real residential 100

square meters house where two young working persons live

(independent building located in Rome: 41�5502100 Nord,

12�30’28" East, 56 AMSL). In July/September 2008 energy effi-

ciency measures (efficient electric appliances, house insu-

lation, radiant heating systems, absorbers, etc) and RES

technologies (photovoltaic and solar thermal plants) have

been applied. To analyze the integration of these technologies,

electrical and thermal loads before (2007/2008) and after

(2008/2009) the implementation are showed. Then the

dimensioning of the photovoltaic and solar thermal plants is

outlined. Finally, the hydrogen backup system (electrolyzer,

metal hydrides and fuel cell) is analyzed, taking into consid-

eration its dynamic behavior, experimentally determined,

giving, in the conclusions, some remarks. The paper not

shows in detail the technologies, because are already available

in themarket, but focus on the integration of the technologies

highlighting some important points.

1.2. Electric loads

The annual electric loads (power and relative electric energy

consumptions) vary during seasons (summer/winter), weeks

and days (different sunset and sunshine so different lights

use, different running time due to the different use of the

house during the week, etc) requiring a per-second registered

power flow during all the year. Anyway, owing to the fact that

the great power variation are during the days, and that the

electric house devices are generally functioning for at least

15 min [4], a power flow over a mean day divided in 15 min

here is taken as representative to analyze the electric energy

loads. Thus taking into account the average power (measured

for a functioning time by a power meter) of all electric devices

(lamps, electrical appliances, pumps, etc), the mean use

during the day and the monthly electrical consumptions

registered, it was calculated the power flow every 15 min of

the mean representative day (estimating the running time

over the day of all electric devices).

Before the implementation of the energy efficiency

measures, due to the use in the summer of a heat pump

(3.5 EER: Energy Efficiency Ratio) to cool the house (during

winter the heating needs are supplied by a natural gas boiler),

two different days (mean summer and winter days) are taken

as representative of the consumptions during all the year. The

energy efficiency measures have been constituted by sub-

stituting incandescent with fluorescent lamps and normal

with efficient electric appliances. Also the heat pump was

substituted with absorbers, avoiding the conversion of the

cooling power demand from thermal to electrical energy. The

substitution of the heat pump with absorbers was made

because in this case the electricity generation is made from

photovoltaic. So even if the heat pump represents a heat

generation from electricity with high efficiency, the global

efficiency of solar thermal plus absorbers is bigger than the

global efficiency of photovoltaic plus heat pump. Indeed the

3.50 EER of the heat pump, greater than the 0.65 EER of

absorbers, does not compensate the low photovoltaic effi-

ciency, 10%, respect to the solar thermal efficiency, 70%,

requiring too extensive photovoltaic (global efficiency of 35%

versus 45%). This is also more true in the heating production

where the COP of absorbers is 1.3 and the solar thermal effi-

ciency is 65%, thus a global efficiency of 85%. So after the

implementation of the energy efficiency measures only one

mean day, showed in Fig. 2, is taken as representative of the

consumption during all the year.

The annual electrical consumptions registered before and

after the implementation of the energy efficiencymeasures are

5000 kWh (13.7 kWh/day) and 1200 kWh (3.3 kWh/day),

respectively. The maximum power requested in the mean day

before and after the implementation of the energy efficiency

measureswascalculatedas4.4 kWand1.7 kWrespectively. The

decrease inpowerandenergydependsmainlyon theremovalof

the heat pump (2.4 kW) and the substitution of the more used

lamps (outdoor, etc) and electric appliances (refrigerator, etc)

owing to the greater power and annual functioning time. The

cost of fluorescent lamps was 34 € (17 lamps, mean differential

price from incandescent and fluorescent lamps 2 €), the cost of

efficientelectric applianceswas500€ (5 appliances: refrigerator,

circulating pump, oven, washing machine, television; mean

differential price from normal and efficient electric appliance

100 €), the cost of absorbers was 5000 € (differential price from

2.4 kW heat pump and absorbers). The payback time varies

between less than one year and ten years depending on specific

annual functioning hours and differential price (mean elec-

tricity price 0.15 €/kWh, mean annual energy saving through

a lamps 50 kWh, through an appliance 100 kWh).

1.3. Thermal loads

The thermal loads were calculated simulating the house in

MC4 software (the house is an isolated rectangular building of

8.76 m width, 13.93 m length, 3.3 m height; the long side are

exposed to north and south, the short side to west and east;

Fig. 2 e Estimated mean day electric power loads.

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the windows area exposition are 0.4 m2 south, 3.81 m2 west,

3.06 m2 east and 2.02 m2 north). The annual thermal loads

(power and relative thermal energy consumptions tomaintain

the temperature in the house at 20 �C) vary during months,

week and day (different solar gain, different external

temperatures, etc) requiring a simulation of the power flow

per second during all the year (as MC4 simulates). Anyway,

owing to the fact that the great energy variation are due to the

different external temperatures among the months [4],

a mean day energy consumption of each month here is taken

as representative to show the thermal energy consumptions.

The Fig. 3 shows the thermal energy needs (kWh) before the

implementation of the energy efficiency measures (the

summers thermal needs are quoted but, as explained, being

supplied by a heat pump, are to be considered as thermal

output of heat pump, so already considered in the electric

loads before mentioned).

The energy efficiency measures have been constituted by:

insulation of the walls, roof and floor (in average the overall

heat transfer coefficient decreases from 1.09 W/m2 K to

0.45 W/m2 K); substitution of the normal windows with low

emission windows (in average the overall heat transfer coef-

ficient decrease from 5.87 W/m2 K to 2.83 W/m2 K); substitu-

tion of the radiators with radiant heating systems (allowing to

fully exploit the solar thermal energy). The Fig. 4 shows the

Fig. 3 e Estimated old mean daily thermal needs.

thermal energy needs (kWh) after the implementation of the

energy efficiency measures.

The annual thermal energy consumptions were estimated

in 14,600 kWh (excluding the summer: 7800 kWh). After the

implementation of the energy efficiency measures the annual

thermal energy consumptions decrease to 4500 kWh

(excluding the summer: 2500 kWh). While the maximum

power requested before and after the implementation of the

energy efficiency measures was calculated as 5.2 kW and

1.6 kW respectively (requested to cool the house during a final

day of July at 18:00). The decrease in power and energy

depends mainly on the insulations of the roof and the floor

and on the better control of the air exchange. The cost of the

insulation of the house was about 15,000 € (6000 € material,

9000 € installation), the cost of the radiant heating systems

was about 7500 € (3000 € panels, tubes, valves, manifold, etc;

1500 € dehumidifier; 3000 € installation).

Finally taking into account the absorbers COP heating of

1.35 and cooling of 0.65, the annual thermal energy needs little

increase (4929 kWh) but in this way, not only it is possible to

supply the cool demand via solar thermal plants, but also the

summer needs (3077 kWh), becoming greater than the winter

needs (1852 kWh), better match the production of the solar

plants. The thermal absorbers energy needs (kWh) are quoted

in Fig. 5.

Fig. 4 e Estimated new mean daily thermal needs.

Fig. 5 e Estimated new mean daily thermal needs

considering absorbers.

Table 1 e Series HBOND 1500 specifications.

Amount of hydrogen stored 1500 NL

Height 284 mm

Diameter 145 mm

Weight 13 kg (including 10 kg metal hydride)

Material aluminum alloy Type ENAA-6063

Hydride LaNi5Heat exchanger inside Al tube: diameter 10 mm, length 6 m

Safety valve 20 bar

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2. Electricity and thermal production

The peculiar characteristic of an Efficiency, RES and Hydrogen

integrated system is in the possibility to fully exploit local

resources to satisfy all the energy needs via energy efficiency,

RES and hydrogen technologies implementation. In this case,

different from [5], the Renewable Energy Sources is the solar

irradiation used via photovoltaic and solar thermal plants. To

calculate the global solar radiation during the day and to take

the average value for every month of the year Lyu and Jordan

method [6] has been considered. Taking into account the

electric energy annually required (1200 kWh), the integration

of the photovoltaic panels in the roof (inclination: 10�; orien-tation: 0�, i.e. south), and an overall plant efficiency of 10%,

a global surface (20% more than the active panel surface) of

about 9 m2 is obtained. Thus five panels of Trina Solar (model

TSM-240DC05) and an inverter of SMA Solar Technology

(model Sunny Boy 1200) was chosen with an overall cost,

including installation, of 6000 €.

In similar way taking into account the thermal energy

annually required (4929kWhplus 766 kWh toheat the domestic

hot water, corresponding of a consumption of 50 l/day of water

at 45 �C, assuming 10 �C as the mean inlet temperature), the

integration of the solar thermal panels in the roof (inclination:

10�; orientation: 0�, i.e. south), and an overall plant efficiency of

60% (the solar thermal plant is composed by vacuum panels,

pump, membrane expansion vessel, and 600 l tank with heat

exchanger), a global surface (30% more than the active panel

surface) of about 8m2 is obtained. Thus a solar thermal plant of

Kloben(modelNRG600)waschosenwithapriceof7300€andan

installation cost of 2500 €.

Fig. 6 e Experimental tank dimensioning graph.

3. Hydrogen backup systems

The system is constituted of an electrolyzer (able to split,

using electricity, water into its two constituent elements, H2

and O2), hydride metal alloy tanks (that absorbs and desorbs

hydrogen) and a polymeric fuel cell (that, recombining H2 and

O2, generates electricity and heat). The backup system is

designed [7] to satisfy the electric load of the mean day

showed in Fig. 2. The characteristic of the tank experimented

is described in Table 1 [8].

The design is done using a graph that describes the exper-

imental tank performances. The graph, showed in Fig. 6, is

made by experimental tests of hydrogen desorption imposing

different constant flow and fitting via equation in similar way

of what done in [9,10]. The measurement system, showed in

Fig. 7, has been constituted of a Sievert system where a Field-

Point, controlled by Lab-VIEW software, manages the flow

meter and detects flow, pressure and temperature of the

hydrides inside the tank during charge and discharge phases.

The graph allows to obtain the necessary discharge flow of the

tank and the remaining time (considering the flow constant)

from the power needed and choosing the fuel cell efficiency.

In fact taking into account only the characteristics shown

in Table 1, assuming a fuel cell efficiency of 50%, the number

of tanks, which contain 6.6 kWh of energy, is equal to 2.

Taking into account the dynamic discharge behavior of the

tanks, however, the number of them may be different and in

particular higher. Taking into account desorption dynamic

behavior [11], not all the hydrogen can be desorbed and the

relationship between the hydrogen released and the mainte-

nance time is not proportional. So each power request (and so

each release of hydrogen) has to take into account the

previous history of the tank, giving an equivalent running

time. E.g. if we need 250W for 75min and 510W for 15min (i.e.

0.694 kWh and 0.283 kWh, considering a fuel cell efficiency of

45%), we can think to use one of these commercial tanks

(1500 Nl of Hydrogen is equal to 4.5 kWh, thus superior to

0.977 kWh needed). Using the graph, for 250 W and a fuel cell

efficiency of 45%, we found that the tank can dispense 3.1 Nl/

min (556W) for 145min (greater than the required 75), and, for

Fig. 7 e Experimental measurement system.

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510 W, 6.3 Nl/min (1133 W) for 45 min (greater than the

required 15). Considering that the tank’s ability to deliver

hydrogen at constant flow for a certain period depends on the

charge level in that moment, when we required 6.3 Nl/min for

45min, i.e. 94 Nl, we have already consumed 231Nl, so it is like

we required 325 Nl at 6.3 Nl/min, so for 52 min, equivalent

time. Thus we have to use two thanks because that thank can

guarantee only 45 min at 6.3 Nl/min. Implementing this

procedure by software, a tank system dimensioning has been

done for the electric load of the case study considered (Fig. 2).

The storage system, considering a fuel cell efficiency of 45%,

results to be of four tanks working in parallel way. Further-

more, the number of tanks is a function of what time of day is

taken as the starting point in the calculation procedure. In

fact, if the initial moment is taken as midnight as in Fig. 2, the

number of tanks is equal to 4, but if the starting point is

instead moved to 17:45 (the starting point of the greater

consumption), the number of tanks is 3. This is because the

storage system can provide more power and therefore

a higher flow for a longer time when it has more hydrogen.

Finally, considering a global electrolyzer efficiency of 70%,

the backup system global efficiency is about 32%. The poly-

meric fuel cell cost is 4500 € per kW, the hydrides cost is 1800 €

per tank (1 € per Nl of hydrogen stored for hydrides plus 20%

for tank cost), the alkaline electrolyzer cost is 2200 € per kW.

Thus the global cost of the system is 17,300 €.

4. Conclusions

The paper shows that with the integrated implementation of

energy efficiency and Renewables technologies it is possible to

reduce to about a third the maximum power and the overall

energy consumptions of a 100 square meters, 2 persons real

house and to supply all the energy needs by small renewable

energy plants (17 square meters is the 15% of the available roof

surface). Moreover the paper shows that electrolyzer, hydride

metal alloy tanks and polymeric fuel cell can realize a safety,

small and reliable electricity backup system, that can satisfy all

the electric needs for one day, if the storage tanks are dimen-

sioned taking into account its dynamic behavior. The hydrogen

system can guarantee more than batteries the functioning over

the years, requiring less maintenance and less weight and

volume. The low-pressure hydrogen storage system through

metal hydrides guarantees system safety too. Moreover the

systemmodularity canalsosatisfyhigherenergy consumptions.

From the economic point of view, while for Renewables and

energyefficiencytechnologiesthepaybackperiodcanbe“short”

(mainlydue to thegreenelectricity feed-in tariff and to the taxes

deduction), for the hydrogen technologies there aren’t incen-

tives and the system installation cost is more than 10 times

greater than traditional system (batteries or generator).

However the “eternal” storage of the system better shows

the advantage of hydrogen. If we considered, for the case

study, a standalone plant (e.g. number of cloudy days: 15;

maximum period of complete cloudiness: 3 days), given the

phenomena of self-discharge, batteries should be sized in

the month of December in which the solar incidence is lower.

The result is a large batteries park that needs a greater

photovoltaic system than the one needed in the case of the use

of a hydrogen storage system (dimensioned over the “year”).

In perspective the development through integrated appli-

cation of hydrogen systems, together with a reduction of

investment cost, can not only increase energy security, but

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above all guarantee net security allowing net metering also at

“house” level.

Acknowledgments

The authors wish to thank their scientific mentors: Professor

Vincenzo Naso and Professor Fabio Orecchini; and the thesis

student Giulia Marinello for their essential contribution. This

two years research has been supported by Region Lazio

(Environment and Energy directorate), in the framework of the

Programme “Polo Idrogeno Lazio” (2006e2009).

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