Reefer containers or refrigerated containers are shipping containers that are used for intermodal freight transport, which require refrigeration of temperature controlled cargo. Reefer containers have integrated refrigeration units that runs from an external electric power supplies from land based sites, container ships or from quays. When transportation is over land/road the reefer containers will be powered from diesel powered generators, which are hooked up to the container while they are travelling. Some reefer containers are water cooled, and these systems are used when the reefer is stored in a vessel without adequate ventilation. Another type of refrigeration uses CO2 for cooling. Reefer containers using CO2 cooling systems are able to keep the cargo frozen solid for up to 30 days. The impact of reefer containers on society is huge, and consumers all over the world enjoy the benefits of fresh produce as a result of them.