MacDonald, who is a non-executive director at the firm, snapped up 500,000 shares at 16.5p each, taking his stake up to 24 million shares, or 18.13 per cent.
Other shareholders in the company – which is building a waste-to-heat plant at Rothes, in Moray, which will run on by-products from whisky distillers – include shipping heir Alastair Salvesen, who owns a 23 per cent stake.