Space Cooling  »  Thermally driven and solar air conditioning

Primary energy savings can also be increased by using the thermally driven chiller for heating in winter and cooling in summer, as the COP for heating of thermally driven chillers exceeds one (Henning, 2010). Primary energy savings in offices in Mediterranean and Southern Europe climates can range from 30 to 53%, for instance (Balaras, 2007). Increasing the share of solar heat used for driving the thermally driven chiller will also generally help boost the primary energy savings. However, the real benefits of using thermally driven chillers comes from the very large CO₂ emissions reductions that are possible in countries with CO₂ intensive electricity generation systems. Given that with the use of solar collectors to provide the thermal energy to drive the chiller the only emissions come from auxiliary electricity consumption. Besides the COP of an absorption chiller, enegy savings obtained compared to an electric chiller depends on the following factors:

• Efficiency of production and transmission of electricity
• Cost of electricity and demand charges
• Cost of natural gas in case of direct fired chillers
• Quantity, quality, schedule and cost of waste heat available
• Availability and cost of solar thermal energy

Savings in energy costs can be large, with examples of 20% to 45% achieved for solid desiccant systems that have been installed in the service sector (Munters, 2008) (Munters, 2009). The savings are affected by relative energy costs. DEC systems save significant amounts of electricity, but require low-grade thermal energy (Urrutia, 2010). For liquid desiccant systems analysis for the residential sector in six climate zones in the United States suggests that the final consumption of energy could be reduced by as much as 46% or increased by as much as 71% depending on the climate zone. However, even where final energy consumption increases, the high savings in electricity mean that overall primary energy consumption will be reduced by between 29% and 66% (Kozubal et al., 2011). The incremental costs for solid DEC systems for residential use aren’t clear yet, but could be 35% (35kW system) to 60% higher (10.5 kW system) than conventional systems. For a single-family dwelling this represents an incremental cost of around €2 200. However, analysis for 6 climate zones in the United States suggests that the life cycle costs of a liquid DEC system will be lower or around the same as conventional air conditioning systems (Kozubal et al., 2011).

Consider the following example:

In a plant where low-pressure steam is currently being exhausted to the atmosphere, a mechanical chiller with a COP of 4.0 is used 4,000 hours per year (hr/ yr) to produce an average of 300 tons of refrigeration. The cost of electricity at the plant is € 0.12 per kilowatt-hour (kWh). A thermal cooling unit requiring 5,400 pounds per hour of 15-psig steam could replace the mechanical chiller, providing annual electrical cost savings of:

These calculations are based on average values and don’t reflect the range of possible outcomes. In countries with lower COPs for new air conditioners and less efficient electricity production systems than the global average, the case for China and India for instance, thermal chillers can save significant amounts of primary energy.

Thermally driven chillers are significantly more expensive than standard vapur compression air conditioning systems. However, costs per kW come down significantly as the size of the chiller increases.

The total costs of small scale (residential) solar cooling systems, including all components required (including the solar collectors) ranges from € 2,000 to € 5,000/kW. Under the best European conditions and with optimised systems, the most economic solar cooling systems can achieve payback periods of 10 years or less. The following tables give indicative costs for absorption and adsorption chillers in Europe. The costs in the table for small-scale systems are significantly higher than the cost of standard air conditioning systems in Europe (around € 200/kW) and don’t include cold distribution, which is included in non-ducted splits. In 2009 average costs for adsorption cooling packages (including chiller, heat rejection system, pumps and controller; but excluding installation) are between € 1,500 and 3,000/kW (Jakob & Kohlenbach, 2010). The total costs of solar cooling systems, including all components required (including the solar collectors) ranges from € 2,000 to € 5,000/kW. Under the best European conditions and with optimised systems, the most economic solar cooling systems can achieve payback periods of 10 years or less.

Indicative small scale absorption chiller costs in Europe (including delivery and start-up)

Indicative adsorption chiller costs in Europe