Mankind’s need for sustainable technology is increasing with the ever growing demand for and depletion of finite energy reserves. Research into the conversion of sustainable energy resources such as wind, hydro and solar into high quality mechanical, chemical or electrical energy is becoming more significant and of greater urgency in modern society. The research reported in this thesis continues from previous work carried out in the development of a small, low cost solar thermal water pump. The aim of the research was to take a pump previously designed at the University of Canterbury to further develop and refine the system to improve operation, performance, physical and operational design, as well as reliability and maintainability. The earlier design was a novel single acting, double diaphragm pump operating on a thermal cycle similar to a Rankine cycle, dubbed The ‘Modified’ Rankine cycle. Although it was adequate to prove the concept it was inefficient and impractical in design. In order to achieve research goals this previous design was dropped and a new double acting unit developed to give greater performance, smoother flow output, be self starting and self priming and therefore be able to operate unattended. The new design is intended to overcome the major failings of the first prototype while being significantly smaller, lighter, more robust and reliable, and be relatively maintenance free. A complex computer simulation of the collector and pump system was modified and developed for the new design to optimise pump geometry and predict pump operation for any average day at any location and for any water head. Thermodynamic analysis of the pump showed significant exergy loss with the internal energy of the working fluid not being used but was an unavoidable trade off for simplicity, cost and reliability. The new pump was optimised to pump water through a 6 metre vertical water head, operating on a 2.9 m² flat plate sheet and tube non tracking solar collector. With n-Pentane as the working fluid operating at 56°C a peak flow rate of 27 litres per minute was predicted with a corresponding efficiency of 0.93%. For a good summer’s day 10,000 litres could be lifted through 6 metres. A suitable initial application of this solar thermal pumping unit would be in solar water heating installations for swimming pools. As a circulation pump a solar thermal unit would have distinct advantages: it would have no running cost, it would add heat to the pumped water reducing the area of water heating panel, and would only pump during daylight hours ie. fully automated and self contained operation.