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In this paper the authors suggest methods to express energy
performance of interior lighting systems and to define an
energy certification procedure.
The authors want to highlight the double utility of the practical
procedures provided [1,2]: indeed, the evaluation of energy
performance on interior lighting systems is certainly important
for energy certification of buildings, but it is so much
important to guide engineers or designers in general, during
design’s course, about three type of choices, to achieve the
results of a comprehensive ecodesign:
1) Luminaires: lamps (tubular, compact, standard, high
efficiency, high output); ballasts (magnetic, electronic,
dimmerable); optical types (direct, semi-indirect, general
diffuse, semi-indirect, indirect).
2) Luminaires arrangements: system’s architecture, number
of luminaires, number of lamps per luminaire, power
needed for lamp and ballast operation, surface of the
room or area.
3) Lighting control system: occupancy and daylight
sensibility, type of the control (manual or automatic),
following skills of the reference maintained illuminance
value.
II. ENERGY PERFORMANCE INDICATORS
A. Luminaire energy performance EPL
Luminaire energy performance can be evaluated by the ratio
between the theoretical electric power density GT [W/m2
]
consumed by the whole lighting system and the selected
reference illuminance value E [lux] to be maintained on the
work plane [3,4]:
EPL=GT/E [W/lm] (1)
The theoretical electric power density is equal to:
GT= Eڄ)1+pa)/ (HLڄCU) [W/m2
] (2)
Where:
- HL [lm/W] is the lamp efficiency;
- pa [p.u.] is the power needed for ballast’s operation;
- CU [p.u.] is the optical type factor.
The maintained reference illuminance value E is defined as
the value below which the average illuminance on the
specified surface is not allowed to fall. The recommended
illuminance typical values are 100 – 150 – 200 – 300 – 500 –
750 lux [5].
Normalization of the theoretical power density with selected
reference illuminance value, allows to have an indicator which
refers just to luminaire quality, independently from the
specific illuminance value fixed during design’s course.
The illuminance initially provided by a lighting installation
will decrease gradually during use due to a reduction in the
lamp lumens, failing lamps, and the accumulation of dirt on
the lamps, luminaires and room surfaces. However, the
illuminance can be maintained at or above the minimum
permitted value E, by cleaning the lighting equipment and
room surfaces and replacing failed and tired lamps at suitable
intervals according to an agreed-upon maintenance schedule
identified by a specific maintenance factor MF.
This factor is the ratio of the illuminance produced by the
luminaire at the end of a maintenance period to the
illuminance produced by the luminaire when new and it takes
into account the overall depreciation caused by the various
events already described in this section:
MF=LLMFڄLSFڄLMFڄRSMF [p.u.] (3)
Where:
- LLMF = lamp lumen maintenance factor;
- LSF = lamp survival factor;
- LMF = luminaire maintenance factor;
- RSMF = room surface maintenance factor.
These various factors can be obtained by reference to Tables I,
II and III.
Therefore, considering MF in equation (2), it’s possible to
obtain a theoretical power density that takes into account the
maintenance operation:
GT= Eڄ)1+pa)/ (HLڄCUڄMF) [W/m2
] (4)
By equation (4) it’s possible to simplify equation (1) and
define a further luminaire energy performance indicator that
takes into account also the management of luminaires and in
particular the maintenance strategy adopted relating to real
work conditions in the time:
EPL= (1+pa)/(HLڄCUڄMF) [W/lm] (5)
lighting system and the selected reference illuminance value E [lux] to be maintained on the work plane: EPS= GR / E [W/lm] (6) The GR is the actual power density of the lighting system. It takes into account the number of luminaires nA and their real arrangement in the room or area characterized of a surface equal to S [m2 ]. Moreover it considers the number of lamps per luminaire nL, the power needed for ballast’s operation pa [p.u.] and the single lamp power PL [W]. GR can be calculated by the following expression: GR = PL/S = nAڄnLڄPLڄ)1+pa)/ S [W/m2 ] (7) System energy performance indicator EPS allows to evaluate over illuminance value achieved on the work plane of the room or zone considered, that is due to the real arrangement of luminaires and also to the power size of the chosen luminaire, according to the commercial series’ values. C. Control energy performance EPC Control energy performance can be evaluated by an efficacy indicator according to the standard EN 15193 [6]. It could be considered as a BAC factor of standard EN 15232 [7] for lighting systems, neglecting parasitic power needed to charge circuit of emergency lighting luminaire and consumed by the standby control system controlling the luminaires when the lamps are not operating. It takes into account control’s system abilities to exploit daylight availability, real room occupancy and to follow the reference illuminance value selected for the task area. EPC can be calculated by the following expression that is a simplification of EN15193 methodology [6]: EPC = FCڄFOڄ]pڄWڄ)FD-1)+1] [p.u.] (8) Where: - FC is the constant illuminance factor. It is a factor relating to the usage of the total installed power when constant illuminance control is in operation in the room or zone. FC factor depends on maintenance factor MF. It could be assumed equal to (1+MF)/2 [6]. - FO is the occupancy dependency factor. It is defined as a factor relating the usage of the total installed lighting power to occupancy period in a room or zone. FO factor depends on the absence factor FA, which is a proportion of the time that the space is unoccupied, and it depends on other factor FOC [6], which is lighting system’s control sensitivity to the real occupancy area.
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lighting system and the selected reference illuminance value E [lux] to be maintained on the work plane: EPS= GR / E [W/lm] (6) The GR is the actual power density of the lighting system. It takes into account the number of luminaires nA and their real arrangement in the room or area characterized of a surface equal to S [m2 ]. Moreover it considers the number of lamps per luminaire nL, the power needed for ballast’s operation pa [p.u.] and the single lamp power PL [W]. GR can be calculated by the following expression: GR = PL/S = nAڄnLڄPLڄ)1+pa)/ S [W/m2 ] (7) System energy performance indicator EPS allows to evaluate over illuminance value achieved on the work plane of the room or zone considered, that is due to the real arrangement of luminaires and also to the power size of the chosen luminaire, according to the commercial series’ values. C. Control energy performance EPC Control energy performance can be evaluated by an efficacy indicator according to the standard EN 15193 [6]. It could be considered as a BAC factor of standard EN 15232 [7] for lighting systems, neglecting parasitic power needed to charge circuit of emergency lighting luminaire and consumed by the standby control system controlling the luminaires when the lamps are not operating. It takes into account control’s system abilities to exploit daylight availability, real room occupancy and to follow the reference illuminance value selected for the task area. EPC can be calculated by the following expression that is a simplification of EN15193 methodology [6]: EPC = FCڄFOڄ]pڄWڄ)FD-1)+1] [p.u.] (8) Where: - FC is the constant illuminance factor. It is a factor relating to the usage of the total installed power when constant illuminance control is in operation in the room or zone. FC factor depends on maintenance factor MF. It could be assumed equal to (1+MF)/2 [6]. - FO is the occupancy dependency factor. It is defined as a factor relating the usage of the total installed lighting power to occupancy period in a room or zone. FO factor depends on the absence factor FA, which is a proportion of the time that the space is unoccupied, and it depends on other factor FOC [6], which is lighting system’s control sensitivity to the real occupancy area.
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