Under a project contracted from the Japanese ministry of Environment, Mitsubishi Hitachi Power Systems has started verification testing of the solar and thermal energy collecting performance of a unique two-stage solar thermal power system.

The tests are being carried out at dedicated, newly completed facilities within the MHPS Yokohama works and will be conducted through to March 2017 as part of a FY2016 project under the ministry’s Low Carbon Technology Research and Development Programme. The project calls for verification of two major features of the system: the efficiency increase conferred by the use, as the working fluid, of higher temperature steam than that produced by earlier systems; and the optimal control of the high-temperature thermal energy storage system. Development of these and other new technologies for the system is hoped to advance the achievement of stable power supplies at lower cost.

The unusual CSP system under test is a two stage hybrid of proprietary design. It combines a low-temperature Fresnel evaporator which collects sunlight while changing the angles of multiple mirror surfaces arranged on a plane, with a tower superheater which collects sunlight via heliostats. This arrangement is said to produce higher temperature steam at lower cost than earlier CSP systems. Approximately 70 % of all the collected insolation goes to the Fresnel evaporator – which is a low cost component – and the remainder is collected by the superheater.

The completed testing facility covers an area of approximately 10 000 square metres, in which are located the evaporator, the superheater, and 150 heliostats that track the sun’s movements and use mirrors to reflect the sunlight to the focus point on the superheater. If connected to a generator, this system has the capacity to produce 300 kWe.

MHPS’ primary aim is to establish low-cost and practical CSP technologies through the combination of a hybrid sunlight collection system and high-temperature thermal energy storage technologies. Hitherto Japanese solar thinking has strongly favoured PV collectors, but this new method is seen to offer a number of advantages. It exhibits less fluctuation in power output relative to fluctuations in solar radiation intensity, and a stable power output, even under cloudy or night time conditions, owing to the thermal mass of the system and the amplification of that by thermal storage. Challenges still needing to be addressed include the greater complexity of the CSP system and relatively higher equipment costs compared to photovoltaic power generation.

At the testing facility, verification of the hybrid sunlight collection system’s technology will be carried out as follows. In the first stage the low-temperature Fresnel evaporator will heat water to produce steam at a near-300°C initial temperature. The steam will then be sent to the superheater, installed on a conventional CSP tower, where, by heliostat-based sunlight collection, it will be further heated to 550°C. Testing of the high-temperature thermal energy storage system will get underway in October, primarily to investigate the viability of night time power generation.