Best of both worlds

Society is becoming increasingly environmentally aware, which is driving a need to increase energy efficiency and reduce our carbon footprint. This is stimulating significant advances in green technology, including solid state led lighting and photovoltaic (PV) solar cells. Both technologies are based on photonics – the manipulation and processing of light and its interface with electronics. The latest solid state led lighting is more efficient, more flexible and more reliable than any other lighting technology and hence is capable of reducing a significant portion of our electricity bill without compromising our lighting environment. PV solar cells tap the Sun's energy to generate sustainable electricity. Although operating on opposite sides of the energy balance – one uses electricity, the other generates it – leds and PV solar cells use virtually identical technologies and have much in common: * Both are semiconductor low voltage dc technologies * Both currently cost more than alternatives * Both require different design approaches to and have different engineering constraints from traditional solutions * Both require substantial up front investment with long payback periods * Both can be retrofitted, but work best when integrated into building design or refurbishment * Both are developed in a global market, yet are installed by regionalised industries. Despite their commonality, these technologies have developed independently. Yet the greatest opportunities for reducing energy consumption and carbon emissions come from combining the two. Early developments in this area are being driven by member organisations of the South East Photonics Network (SEPNET), including PhotonStar and Zeta, which have demonstrated the feasibility of the process. Developments in led lighting Photonics has enabled the development of ultra efficient lighting. LEDs are a semiconductor light source first introduced as a practical light component in the early 1960s, with initial leds emitting low intensity red light. Illumination arises from the energy released by the movement of electrons and the colour of the light is determined by the size of the energy gap of the semiconductor. LEDs are low voltage light sources that require a constant dc current to operate optimally. Therefore they need a device that can convert incoming ac power to the proper dc voltage and regulate the current flowing through the led during operation. The key advantage of leds is that less power is required to light a given area. Lighting is a major source of energy use – up to 50% in commercial buildings – so significant savings can be made by increasing the efficiency. While switching to compact fluorescent lamps (CFLs) can reduce lighting bills by 80% compared with incandescent lamps, leds can more than double that saving while offering additional benefits in lighting control. LEDs also have flexibility in colour output and control not available from CFLs, as well as offering lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching and greater durability and reliability. LEDs last much longer as they are unaffected by turning on and off repeatedly and can now be designed to maintain a precise colour balance throughout their life. At the end of its life, an led lamp will be less hazardous to the environment than an incandescent light or CFL, with most parts recyclable or reusable and, critically, no mercury content. Developments in photovoltaics PV uses semiconductors, but this time to convert the Sun's energy into electrical energy. Solar cells produce low voltage dc electricity, which must be converted to at least 240V ac when fed into existing distribution grids or standard 240V appliances. However, according to the Renewable Resource Data Center, this conversion incurs an energy loss of 4 to 12%. PV installations can operate for many years with little maintenance or intervention after their initial set up, so after the initial capital cost of purchasing solar panel equipment, operating costs are extremely low compared to power technologies. Indeed, there are examples of PV installations working in the harshest environments on remote lighthouses which are still fully operational after more than 40 years of service. Integrating the two It is clear that, to increase efficiencies further, there is an argument for designing systems where the electrical energy generated from PV cells is used directly to power led lighting systems. As both technologies use low voltage dc, there is no energy lost in converting the power from dc to ac then back to dc. Instead, the dc can be used directly by leds. PhotonStar and Zeta are examples of British companies meeting the challenges presented by integrating led lights with photovoltaics. PhotonStar has been involved in pioneering studies with the Nottingham HOUSE project to generate a truly sustainable house. In initial studies, power generated using a 24V PV supply was connected to a CC driver to power a 6.6W led fitting. This system proved to be 93% efficient. Whereas using a traditional 24V dc supply converted to 240V ac to power control gear and an 18W CFL was only 72% efficient. In the project, a three bedroom house was illuminated using 13 PhotonStar EoStar 6.6W LEC recessed luminaires and 13 PhotonStar CeilingStar5SM 7W LED surface mounted luminaires. The total initial cost was £1794, providing a fitting life of 50,000 hours. When the same house was illuminated using CFLs the cost was £1690 for a lamp life of 10,000 hours. The total power required for the LED solutions was 356W, whereas the CFL solution required 1009W. The study has clearly shown that powering led lighting directly from PV has significant benefits and is more energy efficient, with lower overall power and storage equipment cost and 90% less embedded carbon. In another example, Zeta has developed an innovative lighting system for bus stops and bus shelters. A patented curved PV is bonded on a curved bus shelter roof. This charges batteries during the day, which then power a highly efficient LED array during the night. Key to a reliable system which operates throughout the UK's winter is a highly intelligent Energy Management System (EMS). The EMS maintains the charge levels in the batteries so they do not go into deep discharge during the winter. It achieves this by adjusting the lighting level automatically to suit the battery state of charge. The EMS 'learns' what sunlight levels are received and programs the led lighting accordingly. This allows year round reliable operation at optimum lighting levels. This development combines PVs and led lighting to give many benefits, including elimination of light pollution, no delays associated with grid connection, no utility bills, high reliability and a led lifetime of 100,000 hours. In addition, the illuminated waiting areas enhance public safety. In conclusion, photonics is the science behind many of today's energy efficient technologies, such as LEDs and PVs, enabling the next generation of lighting technologies that are more efficient than traditional incandescent lamps. If PVs and LEDs can be integrated, a more sustainable environment can be developed, helping to reduce CO2 levels.