The Duke also travels with police bodyguards and their travel costs are directly funded by the taxpayer.
Critics described the Duke as "totally unaccountable" and expressed astonishment he was once again living up to his nickname of "Air Miles Andy".
The Duke started 2014 with an official three-day visit to Bahrain, quickly followed by his annual trip to the World Economic Forum in Davos, Switzerland.
He was back in Bahrain in April, and clocked up visits to California, Kuwait, Germany, Canada, Afghanistan, China, Saudi Arabia, Switzerland, France, India, Turkey and the United Arab Emirates.
In contrast, the Queen and the Duke of Edinburgh made only one trip abroad last year (to France for the D-Day commemorations) while the Prince of Wales and Prince Edward went to nine countries each, Prince William four and Princess Anne seven.
During some of the trips the Duke, a fanatical golfer, had several days without any official engagements, suggesting he was combining work and leisure. He has been criticised in the past for tagging on golf breaks to official visits funded by the taxpayer.
The palace said that of the 15 countries visited by the Duke, seven were official visits on behalf of the Government and approved by the Royal Visits Committee. They were: Bahrain (January), Kuwait, Afghanistan, Switzerland/France, Saudi Arabia and Germany. Six others, to Davos, California, Canada, China, India and Turkey, were "working visits" paid for out of private funds, while a visit to Bahrain in April and a visit to the UAE were "private" visits.
Labour MP Paul Flynn said: "He is accountable to nobody and if he is doing this using public funds there should be more information about what he is doing and what successes he is delivering."
Andy Silvester, campaign director of the Taxpayers' Alliance, said: "Taxpayers are entitled to ask quite why the Grand Old Duke of York has picked up so many air miles ... It's time for more transparency from the palace about the nature and need of these working visits abroad, especially when you consider the associated costs of his security detail and so on."