HVAC facilities takes up a total electricity consumption of a building by 40%. In Hong Kong, the figures even jump to 60% because of hot weather. While there are few organizations who are conscious of optimizing their facilities for the best possible performance and energy usage, most of them simply aren’t aware of of the fact that Chiller System Optimization and design of the whole chiller plant system may do great help.
With its expertise and experiences in HVAC engineering over the past decades, BOCA developed and proposed a new prospective design: Ultra High Efficiency BocaPCM-TES VSD Hybrid Power Chiller Plant System, to provide a better answer to the familiar problems in the industry. The viability of this prospective solutions can be gauged and have been proved by our successful local projects and many others around the world.
Running days after days, the modern chiller system design faces a lot of challenges. Some of them are simply universal which are very difficult to tackle with, leaving significant negative effects to the issues of chiller efficiency, energy consumption, maintenance and first cost.
With PCM-TES, BOCA is able to mitigate the problems of the chiller plants in HVAC system design in a very effective and unique way.
Chillers have to respond to instant call for cooling capacity so the system tends to be designed according to the highest demand estimated in prime hours plus a large safety buffer, and in reality, this buffer could be a doubling of the required loads. Therefore, chiller plants tend to be installed to over-capacity. They are running less efficiently than if running at partial-load conditions all the time. This causes the problem of energy wastage and increases operating costs for the building owner.
Full of residual cooling capacity in the form of iced PCM panels, BocaPCM-TES is able to act as a “capacitor tank” to store coolness produced all day long for immediate needs in daytime. This tank enables us to reduce the design size of the system. Ordinarily, the ideal load for maximum chiller efficiency is between 60 and 80 percent of design capacity. Our optimized chiller system design also leaves us room to select fit important parts such as coil, control valves, piped coils and tertiary connection and control.
This problem arises when the chiller system design temperature difference between the temperature of chilled water leaving and entering the Chiller systems is not achieved. After mixing with returned water the temperature of the chilled water supplied to building increases, making the control valves remain completely open. The result is that the secondary pumping system will increase energy consumption and either a decrease in chiller efficiency or a failure to meet cooling loads.
Variable-speed drives on chillers dramatically improve part-load performance during low ambient conditions when condensing temperatures can be reduced below design conditions. BOCA designs system which must be able to accommodate low delta-T in an efficient manner while still meeting all coil loads. By using variable-speed-driven chillers, which are so efficient at part load that under all but the lowest load conditions, we also run more chillers than are required to share the load. Thus, additional flow resulting from degrading delta-T will have no impact on chiller energy use.
Coefficient of Performance (COP) is calculated based on full load capacity but in the real world a chiller is hardly always running on full load capacity. Since that 99% of the operating hours for any chiller are on part load conditions, so for evaluating chiller efficiency at different part load condition Non-standard Part Load Value ( NPLV) is calculated.
Performance of chiller is measured at different loading condition, say 100%, 75%, 50% & 25%, to obtain a fair and real performance assessment.
99% of the time, the chiller encounters ambient conditions lower than the chiller system design condition. In such instances, use of compressor having Variable Speed Drive can help achieve higher part load efficiencies. Using Variable Speed Condenser fans can also help in achieving better part load efficiencies.
When designing any energy efficient chiller plant system, part load efficiency must be taken into consideration. Part Load means not only reduced tons of cooling required, but also reduced lift, the difference between evaporator and condenser temperatures, which the compressor must overcome.
As chillers run 99% of the time on part load (off-design) conditions that will reduce compressor lift and internal load of the building, BOCA gives a lot of consideration to part load efficiency while doing your chiller system design.
The Essence of BocaPCM-TES Chiller System Design
Delta means change of an amount or figure. ΔT refers to the change of temperature. In common chiller applications, the temperature of water entering a typical chiller plant at 7℃ and leaving at 12℃. The change of temperature, or ΔT, is 5℃.
Reasons for this relatively low ΔT is that lowering supply temperature increases the work the chiller must do. This increase chiller energy use and peak power. If the leaving air temperature is above 12℃ and the difference between the entering and leaving chilled water is greater than 10℃, you are not getting enough chilled water from the building system. You have to use larger cooling coils, yet common chillers have restrictions from the allowable flow-rate change of the chiller. Other problems include hazards of freezing and possible damage. Engineers have to pay much attentions to freeze protection, water supply and pressure switch limit adjustments.
In an Ultra High Efficiency BocaPCM-TES Chiller Plant System, the chilled water entering the upstream chillers at 16℃ and leaving the downstream chillers at 6℃. The ΔT is as high as 10℃, a double of the common.
Thanks to our PCM-TES, which helps a lot in providing residual coolness capacity, our chillers no longer have to do that much works in order to lower the supply temperature.
The benefits of High ΔT technology are:
(1) Using a larger-than-conventional difference between the entering and leaving chilled water temperatures permits a lower flow rate. Smaller pipes and pumps can then be used to satisfy the same capacity and a lower pumping energy cost, lower pumping peak power and lower tear and wear on the pumps.
(2) The chilled water leaves the chiller at as low as 6℃. This low temperature suffices to solidify, or freeze the BocaPCM panels inside the thermal tank. You don’t spend a cent for a great deal of precious energy to produce the ice in order to store coolness for later use. Remember, this coolness will help reduce the chillers’ total works as well.
While in common practice chillers are usually arranged in a parallel setup, BOCA arranges sets of chillers, upstream and downstream, in a serial configuration. The upstream set of chillers run within a temperature range of 11℃ to 16 ℃, then the chilled water flows through to the downstream set to reach 6℃ from 11℃. This two-tier serial configuration successfully bring our High ΔT technology into real practice.
Because the upstream chillers in serial arrangement operate at a higher chilled water temperature, which means that the refrigerant temperature and refrigerant pressure in the evaporator are also higher in the upstream machine.
Similarly, the downstream chiller will face a lower condenser leaving water temperature and therefore a lower condenser refrigerant pressure than it would have been in a parallel chiller condenser. This reduced chiller powers.
The upstream chiller operates at an elevated temperature and increased efficiency. Very often this compensates for any increase in pump energy. The serial chillers arrangement yields the lowest full-load chiller power (10%+ lower than the parallel–parallel configuration).
For very large chilled water ranges, use series chillers, possibly with series counter flow condenser circuits, to optimize chiller performance.
The reduction in lift (between refrigerant temperature in the evaporator and condenser) provided by the serial chiller arrangement also occurs at part-load conditions. Each set of the chillers in this serial design has its refrigeration circuits thus multiplying the reduction effect of the lift.
When chillers are piped in parallel in a VRF system, there is a significant flow rate change in the operating chiller when the second chiller is added. When the chillers are piped in series, there is no transition flow when the second chiller is enabled. This can greatly simplify system control.
Arranging the chillers in series may also reduce life-cycle costs when compared with traditional parallel arrangements.