Philadelphia-based architects Kieran Timberlake have won the competition to build a new US Embassy in London, with a design that concentrates on environmental issues, Louis Susman, US ambassador to London, announced on Tuesday.

The victorious design, to be built in Nine Elms on the southern bank of the Thames, combines energy efficiency with improvements to the post-industrial landscape of one of London’s last big undeveloped city-centre sites.

It consists of a glass cube sheathed in a stretched, sculptural membrane embedded with gossamer-fine photovoltaic cells. This crystalline second skin both shades the interior from the sun and converts the sun’s rays into power in one of the multiple measures to make this an exemplary green building.

The glazed box sits atop a colonnade intended to create a feeling of civic engagement and openness which has been pointedly lacking in recent US embassies and which became one of the reasons that the state department took the decision to vacate its existing building in central London’s Grosvenor Square.

Amid the residential setting of Mayfair the concrete blockades and the huge architecture of security had seemed hugely intrusive, a reflection of national fear rather than pride.

At the Nine Elms site the architects have produced what seems a sensible and sensitive landscape strategy which obviates the need for such a strident expression of security while attempting to provide superior safety for those working both within and around the building.

A semi-circular pool protects the building on the side facing the river, which features a new road passing between the embassy and the Thames, while the building itself is slightly elevated atop a mound to avoid the possibility of ramming the structure with the car bombs which became such a favoured terrorist mode of attack in the 1990s.

The pool and the grassy mound combine to create a parkland setting which, it is hoped, will both help provide a green heart for the redevelopment of this long-neglected part of the city but which also subtly protects the embassy without the creating the impression of a walled compound – which was one of the fears about the viability of this building as an anchor for the new Nine Elms.

Kieran Timberlake are arguably surprise winners emerging from a final shortlist of four which comprised Pei Cobb Freed & Partners (the practice of I.M. Pei, architect of the Louvre Pyramid and the Museum of Islamic Art in Doha), Richard Meier & Partners (designers of the Los Angeles’ Getty Museum) and Morphosis (architects of the dramatic new Cooper Union building in New York City).

The successful architects have maintained a low profile, eschewing the self-conscious “starchitecture” of many of their contemporaries and have gained a solid reputation as environmentally conscious pragmatists.

The state department described the building as “a beacon that is a respectful icon representing the strength of the US-UK relationship”. It will provide a huge boost for Nine Elms and a further reinforcement of the cultural and development shift to immediately south of the river exemplified by continuing development at the Tate Modern and the latest set of (perhaps over-)ambitious plans for the hulk of Battersea Power Station.

The cost of construction of the new embassy is estimated to be about $500m (€369m, £324m) entirely financed from the sale of the Eero Saarinen-designed 1960 Grosvenor Square building (Britain’s first modern embassy building) to Qatari Diar Real Estate Investment Company in 2009. Outline planning permission for the building has already been granted by Wandsworth Borough Council and groundbreaking on the site is expected to be in 2013 while completion is projected for 2017.