Energy 24 (1999) 681–687 www.elsevier.com/locate/energy Solar collectors versus lamps—a comparison of the energy demand of industrial photochemical processes as exemplified by the production of -caprolactam Karl-Heinz Funken * , Franz-Josef Mu ¨ller, Ju ¨rgen Ortner, Klaus-Ju ¨rgen Riffelmann, Christian Sattler Deutsches Zentrum fu ¨r Luft- und Raumfahrt e.V. (DLR), Solare Energietechnik, D-51170 Ko ¨ln, Germany Received 10 October 1998 Abstract The energy demand of photochemical synthesis of -caprolactam was compared for two plant concepts. The conventional lamp-driven concept followed the process as realized on an industrial scale by Toray Ltd, Japan and a solar concept was designed at identical yearly output. The aim of the comparison was to determine the savings of fossil fuels that could be achieved if photochemistry could make use of solar radiation instead of artificial light. The use of solar radiation for the photochemical production of -caprolac- tam has a 4-fold lower demand for electric current and an 8-fold lower demand for cooling energy as compared to an equivalent conventionally operated route. Furthermore, due to avoided conversion of fossil fuel to electric current, a solar process would allow specific emissions of 1.5–2.5 tons of CO 2 per ton - caprolactam to be avoided, depending on the primary energy carrier used.  1999 Elsevier Science Ltd. All rights reserved. 1. Introduction One of the main problems in using sunlight as an energy source for industrial productions is the discontinuity of sunshine. To implement solar technology, new concepts have to be developed which integrate discontinuous sunlight into a process requiring continuous output of products. Photochemical conversions are candidates for solar chemical processing that deserve particular attention because such conversions can directly follow the availability of light provided that start- * Corresponding author. Tel.: + 49-2203-601-3220; fax: + 49-2203-66900; e-mail: karl-heinz.funken@dlr.de 0360-5442/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII:S0360-5442(99)00024-9