Light Devices

Description

Light devices convert an energy input into electromagnetic radiation within a specific wave-length range: the visible light range. The input energy can be either chemical– in the case of candles or kerosene lamps– or electrical, in the case of LEDs and incandescent light bulbs.

Characterisation

There are currently many technologies available to generate light from electricity. The incandescent, compact fluorescent (CFL), halogen, sodium and Light Emitting Diode (LED) light bulbs. These technologies vary in terms of both efficiency and lighting quality.

Light quality can be quantified using two indicators: Colour Rendering Index (CRI) and the colour temperature (CT). CRI ranges from 0 to 100 and determines the perceived quality of colours compared to solar white light. Values above 80 CRI are needed as a minimum for indoor lighting applications. CT is a measure of the chromaticity of white light, humans tend to prefer lower colour temperatures (i.e. red light) [344].

Incandescent, CFL and halogen light devices are all characterised by relatively lower effi-ciency compared to LEDs and sodium lamps. Sodium lamps can reach very high efficacy values. However, their light quality is much lower than the other technologies and is therefore mostly relegated to outdoor and industrial applications.

The one technology that delivers high efficiency and high quality light is the LED. Moreover, the LED doesn’t have intrinsically low luminous efficiency and its quality can be designed to deliver any light quality. For this reason the LED technology is regarded as the benchmark technology for light devices.

Efficiency measure

The simplest definition of efficiency for a light device is the ratio of electromagnetic energy output over the electrical input. However, this definition is not sufficient because the human eye is sensible only to light with wavelength between 400 nm and 700 nm. Moreover, the intensity of each wavelength varies according to a Gaussian distribution, known as the photoptic luminosity function (v). Therefore the efficiency of light devices is the product of two efficiencies, the electrical to radiation efficiency (η_e) and the spectral efficacy (ε_s)

expressed in lumens per watt (lm/w), where the watt refers to the energy of radiation. The total efficiency of a light device is an efficacy (ε), as it is expressed in units of lumens per watt of input electrical energy.

ε = η_e ε_s (4.31)

The spectral efficacy is calculated as the convolution of the emitted radiation with the photopic luminosity function over the visible wavelength(λ )

εs= Z _∞

0

φrv(λ )dλ (4.32)

The distribution of the emitted radiation determine the overall efficacy, CRI, and CT of the light device. These properties are independent of the electrical-to-radiation efficiency. In fact, η_eis not a good indicator for lighting efficiency. For example, in incandescent lights, η_e is close to 100%, however, the emitted radiation is mostly outside of visible spectrum, meaning that the overall efficacy is limited to around 15 lm/W [345].

Therefore, the metric of overall efficacy is used in this study to define the efficiency of light devices even though it is not a dimensionless energy conversion efficiency.

Key Parameters

LEDs are semiconductor devices that use the electroluminescence mechanism to convert electrical energy into light. The properties of the semiconductors used determine the wave-length (thus the colour) of the light. Current white light LEDs are phosphor coated (pc-LED) meaning that they are composed of a very efficient blue “pump” LED which feeds into a phosphor layer which in turn emits the remaining wavelengths necessary to achieve white light. White light can also be achieved by colour mixing (cm-LED) LEDs of different colours to achieve white light.

The efficiency of LEDs is dependent on the following four lumped parameters

Spectral efficacy The spectral efficiency determines how much of the emitted radiation is percieved as white light by the human eye. The radiation emitted from current LEDs can achieve spectral efficacy values of around 280-320 lm/W with acceptable levels of CRI and CT [345].

Driver efficiency As electrical power in stationary applications is delivered as Alternating Current while LEDs required Direct Current, losses are incurred in the transformation process.

These losses depend on the quality of the current inverters employed.

Photon efficiency Photon losses are incurred in the generation of photons from the elec-trons in the semi-conductor bands. This is a lumped parameter which includes a number of loss mechanisms: internal quantum losses, electrical losses, extraction losses, phosphor losses and Stockes’ losses [346]. These are a function of the LED design and of the chosen semiconductor material and manufacturing quality. Phosphor coated LEDs inherently incur more losses than color-mixing LEDs because they have two extra loss mechanisms (phosphor and Stockes’losses) which occur to transform the blue light into white light.

Optical efficiency Optical losses occur because a share of the produced radiation is not emitted from the device but is absorbed by the device packaging and thermal management components. The optical efficiency is a function of luminaire architecture.

Efficiency limits

The efficiency limits for LEDs found in the literature are reviewed and presented in this section. A parametric model of LED performance cannot be generated using few parame-ters given the complexities of semiconductor physics, therefore the loss reduction method (Methodology B) is used for the estimation of the efficiency limits. The LED literature cus-tomarily refers to mechanism efficiencies rather than losses, therefore the same terminology is used in this section, however the two concepts are interchangeable.

Literature Estimates

In the academic and grey literature, there are a number of predictions of the technical maximum efficiency achievable by LEDs and they are shown in Table 4.7. Not all estimates use the definition of “technical efficiency limit” used in this article as economic constraints are taken into consideration in some estimates, nonetheless it provides a good picture of where expert opinion stands on this issue.

Table 4.7 Estimate of technical maximum efficiency of LED lighting in lm/W Author Year of

esti-mate

Color-mixing? min max

R Haitz 2010 not specified 200 200 [347]

Y Tsao 2010 not specified 200 250 [347]

US DOE 2017 yes 215 283 [348]

E Bretschneider 2007 yes 150 200 [349]

DIAL 2016 no 200 250 [350]

Pimputkar et al 2009 not specified 280 280 [344]

Efficiency Limit estimation

The key loss mechanisms in LEDs are normally referred to as efficiencies which are combined, following equation 4.33, to estimate the overall LED efficacy.

ε = η_d ηWPE ηoεs (4.33)

The technical efficiency limit is estimated by combining the range of maximum values found in the literature for each of the loss mechanisms.

Spectral Efficacy (εs): The maximum spectral efficacy of white light is difficult to establish because it is highly dependent on the light quality [351]. For multi-source LEDs, the DOE believes the maximum efficacy is 414 lm/W [348]. Hatiz et al quote a technical limit of 400 [347], while Murphy [345] quotes a maximum of 348 lm/W. All these values represent what could be achieved while maintaining a CRI of 80. To keep the analysis as conservative as possible, the entire range mentioned in the literature is taken into account for as the technical limit.

Driver Efficiency (ηd): There is no physical limit to the efficiency of AC to DC conversion.

The US DOE has a long term goal of AC/DC conversion for LED luminaires of 95% [348].

The efficiency limit is assumed to range between 92% and 97%.

Wall Plug Efficiency(η_{W PE}): The theoretical efficiency limit of η_{W PE} is a function of temperature, current density, power intensity (W/cm²) and semiconductor design [352, 353].

To achieve LED characteristics typical of space lighting LEDs (∼100 W/cm², ∼50 A/cm²), David et al. estimate that the theoretical limit is 105%. The value is above 1 because the LED acts as heat pump and draws energy from the environment, at very low voltages and current densities, laboratory scale devices with efficiencies up to 230% have been sucesfully tested [354]. Xue has proposed to build a prototype LED with η_{W PE} = 100% for high current densities [355] in the coming years. It is assumed that the technical limit for commercial LEDs is in line with the efficiency of the current laboratory scale light devices, therefore the efficiency is assumed to range between 90% and 100%.

Optical Efficiency (ηo): The theoretical maximum optical efficiency level is 100%. The US DOE has a long term goal of 90% [348] for commercial LED applications. A range of 90% to 95% is considered as the technical limit.

Each of the four sub-efficiencies is modelled as a uniform distribution limited by the values shown in Table 4.8.

Table 4.8 Summary of the assumed efficiency limits of each parameter and the resulting LED efficiency

Device Parameter Efficiency Limits Device Efficiency Limits

LED

η_d 92%-97%

ηW PE 90%-100% 284-350 lm/W

ηo 90%-95%

εs 348 lm/W -414 lm/W

Comparison with the literature

The resulting efficiency limit estimate has a central value of 301 lm/W with a standard deviation of 24 lm/W. This result is higher than all those seen in Table 4.7. There are two reasons that explain this. Firstly, for the estimates in the literature, the technical limit was still referred to commercial LEDs for which there would be a market, therefore those are in fact economic efficiency limits. Secondly, many of literature estimates were made in the period 2007-2011, when commercial LEDs still did not match the performance of halogens and CFCs because of poor η_{W PE} (around 20%) combined with poor colour quality, and technologies such as color-mixing LEDs were not taken into consideration. The estimate provided in this study approaches the theoretical limit of η_{W PE} while still taking into

consideration some unavoidable losses, therefore this is considered a better characterisation of the technical limit of LED efficacy.