University of Edinburgh’s Old College Capital has returned to back energy storage developer Sunamp to take the company’s total to $8.8m.

Sunamp, a UK-based heat battery maker backed by Old College Capital, a VC arm of University of Edinburgh, has attracted £2.2m ($2.8m) in a seed round including gas supplier Osaka Gas, has reported.

Scottish Investment Bank (SIB), the investment arm of government economic development agency Scottish Enterprise, also backed the round, as did investment firm Equity Gap and venture capital firm Par Equity.

Founded in 2006, Sunamp develops compact thermal batteries for household, commercial, industrial and automotive purposes that use a technology billed as energy-efficient, long-lasting and affordable. The batteries can be used for purposes including central heating and hot water.

Sunamp will use the capital to scale its business with a view to expanding in international markets.

Old College Capital backed Sunamp’s $2m funding round in 2015, which was co-led by Par Equity and Equity Gap with participation from SIB, Par Equity, Equity Gap, Highland Venture Capital, and unnamed private investors.

Sunamp later raised $4m in a 2016 round that included Old College Capital, SIB, Par Equity, Equity Gap and Highland Venture Capital.