International efforts to eradicate smallpox in the 1960s and 1970s provided the foundation for efforts to expand immunization programmes, including work to develop immunization supply chains. The need to create a reliable system to keep vaccines cold during the lengthy journey from the manufacturer to the point of use, even in remote areas, was a crucial concern during the early days of the Expanded Programme on Immunization. The vaccine cold chain was deliberately separated from other medical distribution systems to assure timely access to and control of vaccines and injection materials. The story of the early development of the vaccine cold chain shows how a number of challenges were overcome with technological and human resource solutions. For example, the lack of methods to monitor exposure of vaccines to heat during transport and storage led to many innovations, including temperature-sensitive vaccine vial monitors and better methods to record and communicate temperatures in vaccine stores. The need for appropriate equipment to store and transport vaccines in tropical developing countries led to innovations in refrigeration equipment as well as the introduction and widespread adoption of novel high performance vaccine cold-boxes and carriers. New technologies also helped to make injection safer. Underlying this work on technologies and equipment was a major effort to develop the human resources required to manage and implement the immunization supply chain. This included creating foundational policies and a management infrastructure; providing training for managers, health workers, technicians, and others. The vaccine cold chain has contributed to one of the world's public health success stories and provides three priority lessons for future: the vaccine supply chain needs to be integrated with other public health supplies, re-designed for efficiency and effectiveness and work is needed in the longer term to eliminate the need for refrigeration in the supply chain.
COMMUNICATIONshadow loss, and the sheet resistance and absorption losses associated with planar layers that facilitate lateral carrier transport to the grid fi ngers. [ 22,23 ] For high effi ciency silicon heterojunction (HIT) solar cells, contact design requires a trade-off between grid fi nger resistance and the sheet resistance and transmission losses of the transparent conducting oxide (TCO)/ amorphous silicon structures coating the cell front surface. [ 24 ] In this paper, we describe a new front contact design principle that overcomes both shadowing losses and parasitic absorption without reducing the conductivity. By redirecting the scattered light incident on the front contact to the solar cell active absorber layer surface, micrometer-scale triangular crosssection grid fi ngers can perform as effectively transparent and highly conductive front contacts. Previously, researchers have designed light harvesting strings that serve to obliquely refl ect light, which is then redirected into the cell by total internal refl ection from the encapsulation layers. [ 16 ] By contrast our front contact design does not require total internal refl ection at the encapsulation layer. Furthermore in our design, the contact fi ngers are micrometer sized and can be placed very close together such that a TCO with reduced thickness can be used-and in some cases the TCO layer might possibly be omitted completely. We demonstrate with simulations and experimental results that designs utilizing effectively transparent triangular cross-section grid fi ngers rather than conventional front contacts have the potential to provide 99.86% optical transparency while ensuring effi cient lateral transport corresponding to a sheet resistance of 4.8 Ω sq −1 due to their close spacing of only 40 µm. Thus effectively transparent contacts have potential as replacements for both the front grid and TCO layer used, e.g., in HIT solar cells. While related schemes for contacts were envisioned early in the development of photovoltaics technology, [ 25 ] they have not found application in current photovoltaic technology, which is increasingly dominated by high effi ciency silicon photovoltaics. Moreover, the effectively transparent front contact design is conceptually quite general and applicable to almost any other front-contacted solar cell or optoelectronic device. For example, we obtained similar experimental results when applying our structures to InGaP-based solar cells.Figure 1 a,b shows the steady-state electric fi eld magnitude distribution of a freestanding triangular contact and a fl at contact, respectively, with 550 nm monochromatic plane wave illumination normally incident at the top of the simulation cell. For planar grid fi ngers, part of the incident light is refl ected back toward the incidence direction, as is apparent from the high electric fi eld density above the contact plane. By contrast, the triangular cross-section grid fi nger does not exhibit a similar back refl ection, as indicated by the lack of an increased electric
Restricting the light escape angle within a solar cell significantly enhances light trapping, resulting in potentially higher efficiency in thinner cells. Using an improved detailed balance model for silicon and neglecting diffuse light, we calculate an efficiency gain of 3% ab s for an ideal Si cell of 3-μm thickness and the escape angle restricted to 2.767°under AM1.5 direct illumination. Applying the model to current high-efficiency cell technologies, we find that a heterojunction-type device with better surface and contact passivation is better suited to escape angle restriction than a homojunction type device. In these more realistic cell models, we also find that there is little benefit gained by restricting the escape angle to less than 10°. The benefits of combining moderate escape angle restriction with low to moderate concentration offers further efficiency gains. Finally, we consider two potential structures for escape angle restriction: a narrowband graded index optical multilayer and a broadband ray optical structure. The broadband structure, which provides greater angle restriction, allows for higher efficiencies and much thinner cells than the narrowband structure.
HighlightsHigh energy efficiency in storing and delivering vaccines reduced the recurrent costs of distribution.Adopting a system of planned vaccine deliveries by dedicated electric vehicle was more reliable and timely.Solar modules on the roofs of stores linked to the electrical grid, generate enough ‘green’ energy for storage and transport.‘Green’ distribution systems for vaccines and medicines meet an increasing need for cooling in public health programmes.
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