Solar energy meets air conditioning

air conditioning, Klima-Therm, chiller, Gree, SolarPV

A major Chinese manufacturer is demonstrating how to use solar energy to power large-scale air conditioning. Roberto Mallozzi and Tim Mitchell of Klima-Therm look at the significance.

From an environmental and running-cost point, it makes total sense to use the Sun’s energy to power air conditioning. It has the double benefit, of course, of harnessing a free source of power at the time cooling demand is highest — when the Sun is shining.

A number of attempts have been made over the years in pursuit of this Holy Grail for cooling buildings. Until recently, however, the expense and relative lack of efficiency of photo-voltaic (PV) systems has hampered widespread adoption.

A recent development by Chinese manufacturer Gree, however, opens the way to PV-powered air conditioning as a mainstream technology. What is particularly exciting about the advance is that it makes possible widespread adoption of PV-powered systems in even relatively high northern latitudes, such as the UK.

Conventional wisdom has been that PV systems might, at a pinch, be able to power a small distributed split system. Indeed Gree has developed this technology, and can offer such a system from its standard range. However, the company has now taken on the bigger challenge —developing a commercially viable PV-powered centrifugal chiller.

Cooling capacity measured in MW rather than kW brings air conditioning powered by solar energy within reach for large or multi-storey buildings with higher cooling demands. Looking at the number of major buildings planned for construction in London over the next five years, it is easy to appreciate the enormous potential — not to mention the replacement market for aging centrifugal chillers installed in the 1970s and 1980s.

The 2.28 MW demonstration plant at Gree’s in China and the 3890 m2 solar-PV installation that powers it.

The most obvious benefits are the potentially dramatic reductions in running costs and commensurate cuts in carbon emissions. Gree introduced its first PV direct-driven inverter centrifugal chiller last year. With a COP 7.10, it can deliver energy savings of up to 40% compared with a standard centrifugal chiller.

The 2.28 MW demonstration plant at the company’s facility in China has been attracting attention from a steady stream of visitors. After operating for a year from a 3890 m2 PV array, it has saved more than 831 600 kWh of energy, provided free electricity and saved some £93 400 in energy costs.

This is reason enough to pay serious attention to PV cooling. However, the benefits don’t stop there. Electricity not used by the chiller can be fed into the grid, helping further to significantly offset capital costs and providing an ongoing source of revenue for the end user.

The chiller consists of three main components: the photo-voltaic system; an inverter-driven centrifugal chiller and an integrated power control system.

Crucially, DC power generated by the PV is used directly to drive the chiller, under the management of an on-board power-control system. This direct PV power approach increases efficiency by 6 to 8% compared to a conventional grid-connected PV generation plant and air-conditioning system.

The system has five operating modes, with varying components of PV power generation and grid energy consumption, which are matched to current demand and solar conditions.

Gree’s PV-powered chiller is based on five core technologies

1. PV direct drive technology consumption integrated management technology The chiller is driven by DC power directly from the PV generator, overcoming the efficiency losses associated with AC/DC conversion during power upload and power supply.

2. Ternary commutation technology Pioneered by Gree, this enables ternary current inversion between the PV system, centrifugal chiller and power grid. It takes less than 10 ms to switch to any of the system’s five working modes, delivering exceptional operating efficiency.

3. Maximum power point tracking technology MPPT monitors and manages the sometimes unstable voltage generated by the PV power source. When used with AC/DC commutation, it means the system doesn't require the constant voltage modulation required by conventional PV air conditioning.

4. Pulse amplitude-width modulation interleaving control technology Gree’s PAWM interleaving high power, Permasyn motor-driven control manages the dynamic aspects of the centrifugal chiller’s load. It enables the system to adapt frequency and voltage modulation to the prevailing load, to ensure stable operation.

5. Power integrated management technology This integrates with the PV Microgrid and HVAC control to ensure seamless automatic management of cooling demand, PV power and grid power consumption.

• 100% PV energy powering the chiller

• 100% grid energy powering the chiller

• 100% of PV energy being fed in to the grid

• A varying proportion of PV energy powering total chiller demand, with remaining PV energy being fed into the grid

• 100% of PV energy being used by the chiller, with the remaining requirement topped up from the grid.

The switch between each mode takes less than 10 ms, and is virtually seamless, enabling the system to constantly respond to changes in demand and supply, and select the most efficient mode at any one time to satisfy current cooling needs.

With the development of this direct-drive PV centrifugal chiller, we believe the era of large-scale photovoltaic powered air conditioning has arrived. Applications for the technology are numerous, and include large office buildings, data centres, hospitals, schools, shopping centres and the process cooling for industry.

Having seen the scale and quality of Gree’s research and development facilities first hand, we have no doubt about the company’s commitment and ability to make the technology available globally and that it will in due course become a mainstream option.

Gree manufactures a third of air-conditioning units sold in the world today — reflecting growing own-brand sales and its role as an OEM to some of the industry’s leading names.

With the arrival of the PV chiller, and a PV-powered VRF system already available, there can be little doubt that it is intent on making a name for itself in the months and years ahead.

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