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Municipal street lighting retrofit in Bulgaria

1. Introduction been demonstrated to simultaneously reduce ecological
impacts while maintaining efficiency. 10
The judicious utilization of electrical energy in road Real-world case studies, including the retrofit of
lighting is regarded as a pivotal element for urban approximately 3,768 luminaires in Kraków, achieved
sustainability. Public street lighting typically contributes notable reductions in energy consumption and carbon
around 1–1.5% of a country’s electricity consumption emissions, while also delivering improved economic
and can account for up to 30–50% of municipal performance through savings on emissions allowances.
11
energy expenditures, depending on the specific local Similar initiatives in European towns demonstrate
circumstances. 1 consistent compliance with technical standards, safety,
Conventional lighting technologies, such as high- and environmental outcomes. 12
pressure sodium (HPS) and mercury vapor lamps, The findings, when considered collectively,
present several drawbacks. These include low luminous demonstrate that the implementation of LED-based
efficacy (approximately 80–100 lm/W), limited public lighting, in conjunction with adaptive control
operational life (approximately 12,000–24,000 h), and strategies, has the potential to significantly contribute
high maintenance demands. Conversely, light-emitting to the European Union’s 55% greenhouse gas reduction
2
diodes (LEDs) typically achieve luminous efficacies of target by 2030. This integration of lighting and control
at least 150 lm/W, provide service lifetimes of at least systems is expected to yield several key benefits,
50,000 h, and significantly reduce maintenance costs. 3,4 including enhanced operational cost-efficiency,
In accordance with European standards, such as improved road safety, and a reduction in environmental
EN 13201, objective metrics—including power per degradation.
length (PL [W/m]) and power per illuminated area (PA The evolution of technologies in street lighting
[W/m ])—are employed to evaluate the performance of modernization in Bulgaria over the past three decades
2
lighting systems. Retrofit initiatives frequently yield can be divided into two main periods :
5
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energy savings of 50–70%, while intelligent control (i) Transition from mercury to HPS and compact
strategies, such as adaptive dimming, increase total fluorescent lamps (CFLs) (1995–2010).
savings to over 80%. 6 (ii) Transition from HPS lamps and CFL to LEDs
A longitudinal monitoring study in Turin by Valetti (2010–2015).
et al. documented a 51% annual energy reduction The approximate number of lamps of different types
3
following LED retrofits, with photometric degradation installed in outdoor luminaires in Bulgaria in 2010–
being lower than predicted by standard models. Aghemo 2011 is shown in Table 1. 14
et al. reported complementary findings across urban The lamps used for outdoor lighting in Bulgarian
4
lighting installations. The carbon footprint of LED street municipalities at the end of the transition from mercury
lighting systems has been quantitatively assessed, with to sodium and CFL technologies (2010–2011) were as
estimated reductions ranging from 0.4 to 0.7 kg CO / follows :
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2
luminaire/year, depending on operational conditions (i) HPS lamps of 50W, 70W, 100W, 150W, 250W, and
and energy mix sources. These findings highlight the 400W.
7
substantial potential of LED-based lighting solutions to (ii) CFLs of 11W, 13W, 36W, and 55W.
contribute significantly to climate mitigation. (iii) Metal halide lamps of 35W, 70W, 100W, 150W, and
Adaptive smart-city lighting projects—such as those 250W.
integrating the Internet of Things and agent-based
control—have demonstrated up to 35% additional Table 1. Distribution of luminaires by lamp type
energy savings over fixed-schedule designs. Smart in Bulgaria (2010–2011)
dimming strategies linked to predictive traffic flows and
environmental conditions further enhance performance. 8 Lamp type Number Share (%)
Large-scale municipal dimming experiments, such as Sodium high pressure 450,000 53.0
the one conducted in Tucson involving approximately Mercury vapor 136,000 16.0
20,000 luminaires, yielded approximately a 5% Compact fluorescent 222,000 26.2
reduction in zenith sky brightness, corresponding to Metal halide 36,000 4.3
an overall street lighting contribution of around 14%
to urban skyglow. The adoption of warmer white or Light-emitting diode 4,500 0.5
9
amber LEDs within spectral design frameworks has Total 848,500 100

Volume 22 Issue 6 (2025) 181 doi: 10.36922/AJWEP025310242