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2. Akpan, I.E., Sasaki, M. and Endoh, N., Optimization of domestic-size renewable energy system designs suitable for cold climate regions, JSME International Journal, Series B, Vol. 49, No. 4 (2006b), pp. 1241-1252.
3. Database of soil temperature, available from <http://www.chinetsu.jp/zenkoku-chicyu.php> (accessed on 22 July, 2013) (in Japanese).
4. Jyukankyo Research Institute Inc., Handbook of domestic energy 2009, The Energy Conversion Center, Japan, ISBN 978-4-87973-355-9 (2009) (in Japanese).
5. Japan Industrial Standard, JIS R 2616-2001, Testing method for thermal conductivity of insulating fire bricks (2001) (in Japanese).