Volume 9 Issue 3
How is system efficacy calculated for PV lighting systems using various light sources?
When calculating the total efficacy of a photovoltaic (PV) lighting system, take into account the efficacies and/or efficiencies of all of its individual components (Zhou and Narendran 2005). This calculation is relevant to sizing the PV system (see What information is needed to specify a PV lighting system?). One of these components is the light source. When selecting a light source for a stand-alone PV lighting system the following factors should be taken into consideration:
The total system efficacy of a PV lighting system is associated with the efficacy of the light source and the efficiencies of the other system components. The typical luminous efficacies of various light sources are:
Note: LED lamp efficacy has improved rapidly in recent years, and a series of premium white LED products are now achieving efficacies about 35 to 40 lumens per watt (at time of publication).
The formula below can be used to calculate the overall system efficacy for a stand-alone PV lighting system:
E = f / P
f = P*hPV * hbat * hele * Esrc * hlumWe have
E = P*hPV * hbat * hele * Esrc * hlum / P
= hPV * hbat * hele * Esrc * hlumWhere:
hPV: PV panel efficiency, assume hPV = 15%
hbat: Battery efficiency, assume hbat = 80%
hele: Product of efficiencies of all electronics, which may include efficiency of charge controller (hcha), inverter (hinv), and ballast/driver (hdri)
Esrc: Efficacy of light source, as lumens per watt
hlum: Luminaire efficiency, as the ratio of total output lumens from the luminaire to total lumens from the lamps
We assume that hPV= 15%, hcha, = 90%, hbat = 80%, and the battery is large enough to meet the maximum power demand of the system. The PV efficiency of 15% is on the high end, which may be achieved with premium level PV panels available on the market. The battery efficiency of 80% is also on the high end, which implies high-quality batteries and little conduit loss. Table 5 shows the remaining assumptions used to calculate system efficacies for different system configurations.
Note that in this table, CFL and LFL (linear fluorescent lamp) luminaire efficiencies are higher in general lighting applications than in directional lighting applications. This is important to consider because outdoor lighting applications, especially those using low light levels, tend to favor directional lighting and allow a user to aim or focus the light where needed.
To illustrate the calculation listed above, consider the case of a PV lighting system using white LEDs as the light source. The system efficacy in directional lighting applications, expressed as lumens per watt (LPW) of solar power reaching the PV panels, can be calculated as follows:
ELED = hPV * hbat * hcha * hdri * Esrc * hlum= 15% Ã— 80% Ã— 90% Ã— 85% Ã— 25 LPW Ã— 85%
= 2.0 LPW
In comparison, a lighting system using a CFL as the light source does not need a dc current regulator, but it does need a dc-to-ac inverter. The system efficacy can be calculated as follows:
ECFL = hPV * hbat * hcha * hdri * Esrc * hlum= 15% Ã— 80% Ã— 90% Ã— 80% Ã— 65 LPW Ã— 60%
= 3.4 LPW
Note that the efficacy values above were calculated for every watt of solar power that arrives at the PV panels. The ideal solar radiation power is 1000 watts per square meter at sea level, which is approximately 100 watts per square ft.
Using similar calculations, NLPIP obtained the results found in Table 6 (in "What factors should be considered when selecting a luminaire for PV lighting?"), which summarizes the system efficacy (LPW) for PV lighting with different light sources in directional lighting applications. Since LED technology is rapidly evolving, the efficacies of LED are expected to increase significantly over the next 15 years. According to U.S. Department of Energy's roadmap on solid-state technology (U.S. Department of Energy 2005), for example, the efficacy of white LEDs is expected to reach 100 lumens per watt by the year of 2010, and 140 lumens per watt by 2015. Therefore, Table 6 contains projections of system efficacy using white LED lamps in future years.
In the survey conducted by NLPIP (see "What are some common beliefs about PV lighting systems?") respondents indicated that LEDs were the most suitable light sources for PV lighting applications followed by fluorescent lamps, while halogen light sources were considered not suitable for PV lighting applications. However, given current lamp efficacies, PV lighting systems using fluorescent lamps are the most efficient. PV lighting systems using white-light LEDs are considerably more efficient than systems using halogen lamps, but neither is as efficient as those using fluorescent lamps.
In addition to system efficacy, the impact of the outdoor environment on the PV lighting system also needs to be evaluated. For example, even though ac light sources such as fluorescent lamps have a higher luminous efficacy than LEDs or incandescent lamps, they may have starting problems and/or reduced light output in low temperature environments.
Lumen maintenance and lamp life should also be considered when choosing a light source for a PV lighting system. CFLs and LFLs tend to have very good lumen maintenance (up to 90% or greater) and long average rated lives (ranging from 6000 to over 20,000 hours). However, these projections are based on lamps operating under ideal power conditions. Because the power conditions provided by PV lighting systems may vary much more than those provided by grid-powered lighting systems, it may be reasonable to assume that both the life and lumen maintenance of these sources would be reduced. There are no data available to end users concerning the performance of these light sources under PV power conditions, so the risks of using them in PV lighting systems increases. LEDs and incandescent light sources, on the other hand, are generally less affected by variable power conditions than their gas discharge (fluorescent and HID) counterparts.
CFLs and LFLs also have relatively high total lumen output per lamp and are relatively high wattage (e.g., 9 watts or greater). LEDs and incandescent lamps, on the other hand, are available in very low-wattage versions, providing very low light output. White-light LEDs, for example, are available in versions requiring as little as 0.2 watts. These low power requirements and low total light output are well suited to low light level PV lighting applications because they allow for the use of low-mounted luminaires with small PV panels and small battery capacity. Therefore, assuming low light levels are acceptable for the application, PV powered luminaires using these light sources (LEDs or incandescent lamps) are a much more viable option than larger, high-mounted luminaires using CFLs or LFLs.