## 1 Introduction  
Water is an indispensable resource for life. It is considered one of the fundamental rights of people. Its availability is not uniform, but depends on geographical location and climate. In some countries, basic needs, including water availability, are not assured. In July 2010, the United Nations General Assembly recognized the right for everyone to have access to sufficient water for personal and domestic use. Other institutions have also identified other water requirements, such as quantity (50-100 L per person per day; WHO), its cost (not over 3% of household income; United Nations Development Programme) and distance travelled (not over 1 km from home) [1]. Moreover, water quality is important as its quantity: poor quality water causes health problems for about half of the population in developing countries. In other areas, where the quality of life is higher, resource use is often affected by low efficiency and the cost of water is high. Water charges should be met by citizens if public administrations are unable to charge for water supplies. In this situation, more sustainable management of water production and distribution is needed. Several technologies have been studied and used in the past to obtain water from the ground, the air or the sea. The technologies vary depending on community needs and resources. In developed countries with coastal geographies, desalination plants can produce large amounts of water for building use in cities. In the same countries, as well as several other regions, however, air conditioning systems are widespread. Therefore, the use of condensed water from air conditioning systems appears to be of potential interest, in addition to the much more widespread use of seawater desalination. The processes for extracting water from the sea are well known and has been studied for a long time. In contrast, the possibility of recovering water from processes used to produce cold and hot air from HVAC (Heating, Ventilation and Air Conditioning) systems has been only recently considered, partly due to their growing use and spread. Like drinking water, air conditioning is a valuable resource, particularly in hot and dry regions. Statistics suggest 25% of global HVAC systems are in use in Europe, 30% are in North America and 40% are in the Asia-Pacific region. The rest of the world thus accounts for 5%. The global HVAC systems market is expected to experience compound growth of 5.5-5.9% in the 2015-2023 period, due mostly to Europe and North America, with Asia Pacific the largest regional market over the period [2]. Therefore, in hot, dry or humid climates in arid regions where refrigeration processes represent a basic need for indoor comfort, the advantages of an integrated design of HVAC systems optimized for water production, rather than for air treatment, should be evaluated. During the past fifteen years, research interest regarding the use of condensed water from air conditioning systems has increased. This kind of application has been studied mainly in climatic conditions where HVAC systems are widely used for refrigeration. Water production efficiency of an evaporator coil was calculated referring to the hot, humid weather of Jeddah (Saudi Arabia). The simulated production amount was found to strictly depend on air speed through the coil [3]. From the results of a wide-ranging review on air-conditioning condensate recovery and applications, total costs of mechanical systems have been found to be 3-5% higher for integrated systems if appropriate planning is undertaken at the design stage of new buildings [4]. Studied solutions have usually considered water coming from low-quality sources and intended for low-grade uses, such as toilet flushing, irrigation, etc. This is usually because the energy performance target has been considered independent from the interest in obtaining water from these systems.