Solar field freeze protection
At the given site, ambient temperatures fall below 2 °C at 20% of the year . But even other regions for future applications of this technology may require freeze protection for the solar field. The intention to feed the solar steam directly into the existing distribution precludes the options to operate the solar field with refrigerants or antifreeze additives to the water. Another option is to decommission the plant during winter. Draining and refilling the system requires hardly any additional equipment installation. However, maintenance effort will be increased, and annual performance reduced. The latter could be avoided by an automatic draining and refilling system, at the expense of increased complexity and cost.
The proposed solution is circulation heating during times with danger of frost. First estimates indicate that the solar gain during winter will over-compensate the heating demand for freezing protection, subject to verification during the system operation and monitoring phase. Heat sources
for the freeze protection could be either steam or condensate, but also waste heat utilization or electric heating may be considered.
The layout and integration of direct solar steam generation for a process heat application with parabolic trough collectors has been planned for a demonstration plant, which is under construction and will start operation late 2008. This will be the first installation which allows test and evaluation of direct steam generation in an industrial environment. A monitoring program will be performed to validate the design assumptions and simulation models. The integration of a solar steam generator into the steam distribution of existing conventional installations can be a cost effective alternative to the retrofit of solar steam or hot water systems supplying individual low to medium temperature processes.
The authors gratefully acknowledge the financial support given to the P3 project by the Federal German Ministry for the Environment, Nature Conservation and Nuclear Safety (contracts No. 0329609A, 0329609B, and 0329609C).
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