18 August 2019 Sunday

Payback Period Calculations For A Light Tube System


- Barbaros Batur,PhD, Yildiz Technical University
- M. Cem Çelik,PhD, Marmara University
- Muammer Akgün, BACADER (Association of Chimney Manufacturers and Builders)


A light tube is a device that captures outdoor light and redirects it for indoor lighting. They range from complex devices that use fiber optics to practical solutions that reuse plastic bottles. Light tubes commonly have a lens that captures light, a pipe of reflective material and an element for diffusing light into a room. They may be paired with dynamic electronic lighting that dims when natural light is available. In some cases, a light tube also provides controls so that they can be turned off or dimmed.

In this study, the economic analysis and payment period of light tube application in a part of a factory established in Gebze was calculated.

Keywords: Light tube, Energy saving,


The popularity of renewable energy sources are on the rise again due to the growing environmental concerns over the limited supply of fossil fuels. The sun is the main energy source of our planet. Hence, solar energy has the greatest potential among the other renewable sand offers the largest opportunities. The technological developments and the new materials pave the way for the wide spread use of solar energy systems. Solar energy is utilized for more than one ways such as, electricity production, domestic hot water heating, desalination, lighting purposes. On the other hand, the installation cost of a high technology solar system is an important constraint. Calculating the payback period of such systems is a difficult task with the changing material costs, volatile prices of electricity market and variable energy subsidies [1]. 

Example Factory

Turkey has a great solar energy potential. In this study, the technical and economical analysis of a light tube system installed in a factory in Gebze is investigated. The factory operates in various shift schedules due to seasonality in demand variations. There are two shifts for 8 hours in January. From February to August there are three shifts. The production stops during the second half of the August for maintenance and vacation. Due to the decline in sales, only one shift remains from September to December. The shifts are distributed in the day consecutively, the first shift 08:00-16:00, the second shift 16:00-24:00 and the third shift 24:00-08:00,Table-1.

It is assumed that, the light tube system starts an hour after the sunrise and stops an hour before the sunset. Thus, the number of hours of lighting are calculated for each month with the light tube or the conventional system. 

 Table -1: Monthly operation hours for light tube and conventional system 

Consecutively, it is an important task to calculate the average number of sunny days and the amount of light that can be utilized in cloudy days. For the cloudy days, it is assumed that the half of the lighting is supplied from the conventional system. The average number of cloudy days and the days of conventional system usage is demonstrated in Table 2. 

                                                          Table-2: The number of conventional lighting days due to clouds.

In October 2018 1 USD was 5.60 TL and the price of electricity for three period in a day was 0.4011, 0.6282 and 0.2389 TL / kWh [2].

                                                                                 Table-3: Monthly Energy Savings

There are 368 conventional lighting equipments used in the factory. Energy consumption of the conventional lighting equipments for each month is demonstrated in Table-4. 

                                               Table-4: Monthly energy consumption of the conventional lighting equipments



The calculation of the total cost of electricity consumption is evaluated and given in Table-5. 

Table 5 Total Lighting Improvement                                                                    Table-5: Total Lighting Improvement

As demonstrated in Table-5, the annual cost of electricity consumption for lighting is 41,816 USD for the factory. Since the solar tube application provides %44.9 of the energy usage or 18,775 US annual saving in lighting system. As an additional benefit, conventional lighting system provides 113.4 lux, on the other hand, the proposed light tube system provides 250 lux. The installation cost of light tube system is around 90,000 USD [3]. Consequently the payback period can be calculated as:

90,000 US installation cost / 18,775 USD annual savings = 4.79 years

If the light tube is used in such a way that the amount of lighting remains the same; The light tube cost would be 41,440 USD, If the conventional lighting system illumination level 114.3 lux is to be attained.

In this case, the payback period would be, 90,000 installation cost / 41,440USD USD annual savings = 2.17 years. 


[1] Varınca, K. B. ve Gönüllü, M. T., 2006. Türkiye’de Güneş Enerjisi Potansiyeli ve Bu Potansiyelin Kullanım Derecesi, Yöntemi ve Yaygınlığı Üzerine Bir Araştırma, I. Ulusal Güneş ve Hidrojen Enerjisi Kongresi, ESOGÜ, Eskişehir, 21-23 Haziran, s. 270-275. f

[2] http://enerjienstitusu.de/elektrik-fiyatlari/

[3] Factory data