The fall and rise of energy costs
Managing director of Advanced Energy Monitoring Systems Maurice A Yates reveals how the water and wastewater industry can reduce its electricity bills and lower its energy costs
For instance, in the last few years, Ireland has gone from one of the cheapest electricity supply points in Europe to one of the most expensive. The UK is likely to follow this trend. This, along with AMP4, is giving a new impetus to reducing energy consumption and hence costs.
There is a significant difference in considering energy costs in water systems compared with wastewater systems. In water systems it is still the pumping plant that dominates the electricity consumption and is usually in the region of 85-90% of a station's electricity bill.
In the case of wastewater, stricter environmental standards are requiring the use of new processes, such as biological air filtration (BAF) plants. These plants consume considerable quantities of air, which has to be blown into the plant. Even so, pumping is still likely account for more than 50% of the electricity bill. In view of the above it is best if we treat water and wastewater in two different ways. Considering water, my experience is there are several areas of savings opportunities, some of which must be tackled at the design stage. Those designing plants would be well-advised to read Natural Capitalism: Creating the Next Industrial Revolution. Page 116 gives an interesting insight as to how undersized pipework lead to a 13-fold increase in pumping power. When considering pumping plants, one should remember the pump is required to pump not only the static lift or geodetic head, it must also pump the friction content. This friction content is inversely proportional to D5, so increasing pipe diameter by just one, or even two sizes, will have a significant effect on the total pumping plant size, which will reduce running costs. I have seen examples where schemes, if they had a lower friction content, would have been fitted with 200kW pumps at low voltage but because of a high friction content, 500kW pumps were required and a high voltage electrical system had to be installed. Also, at the design stage, it is important to use bends and swept tees instead of right angle connections. Again, for a minimal investment, one can achieve significant energy savings. When designing pumping station pipework the economic pumping velocity is in the region of 1.5-2.0m/sec.
This is also the velocity that should be used through a non-return valve. Hence, as the exit velocity through most pumps is now often in excess of 4m/sec, a diffuser should be fitted immediately to the discharge flange of the pump before any valve work.
Also at the design point, cost-effective transformer selection and cable sizing is often ignored. It should be remembered that the installation regulations are written around a 2.5% voltage drop in the system, this immediately increases the pumping cost by 2.5%. Increasing the cable size is a cost-effective investment to make to save the distribution losses. With existing plants there are several areas where pumping costs can be reduced, these include:
- pump condition - water pumps deteriorate throughout their life and at some point it is economical to refurbish the pump and return it to its original condition. In the case of older pumps, replacement is often the best option. Typical savings are in the region of 12.5%,
- pump duty or selection - there are many instances where the pumping plant is over-designed and occasionally it may be under-designed. Over-design leads to many problems. Firstly, the pump will be running to the right of its best efficiency point and will be running less efficiently. It is most likely to be cavitating, which will reduce pump life and increase both the energy consumption and its wear rate. Dependent upon the specific situation, there are a few things to be considered, such as removing a stage from the pump, trimming the impeller, fitting a variable speed drive and in some cases, fitting new pumps. Savings in this area vary considerably from say 5% to more than 50%. Pumps are designed to run at their best efficiency point (BEP) and are best used in the region of -10% to +5% of this point,
- variable speed drives - the answer to the maiden's prayer? No, while there are many instances when a variable speed drive is the correct answer, there are probably just as many instances when their installation has covered poor basic design. Yes, variable speed drives have a great place in pumping, particularly where the friction loss is greater than 20% of the static head. Here small changes in speed can significantly reduce energy costs without significantly affecting the output flow. It must be remembered though that when a pump's speed is reduced, say below 80% of rated speed, then the pump efficiency is likely to fall off the cliff,
- pump controls and optimisation - here we are considering which pump to run and when and where to use parallel pumping against solo pumping. Firstly, one should select the most efficient pump to run solo but careful selection is required as to when to run a second pump in parallel. It can be dangerous to run a fixed speed pump against a variable speed pump and just as dangerous to run two similar pumps at dissimilar speeds. It is also easy to set up control systems which may see one or more pumps running badly throttled and, in some instances, running against a closed valve. The point of change-over from running one variable speed pump to running two variable speed pumps in parallel is critical. One recent example showed by starting the second pump at a lower flow rate than previously produced a saving of 13%, which amounted to a reduction of £15,000 in the electricity bill. In another multi-pump installation, modifying the pump selection against the flow requirement resulted in an 18% reduction in energy consumption, a saving of £22,000 per year.
Installation effect is an area which is difficult to improve after design. However, there have been instances where replacing the bitumen lining of pipe and bends with modern pipework protection materials have significantly reduced the head loss in the piping system, typical savings are 1-2%. Energy management are just two words unless one puts in place an effective monitoring and targeting system. On smaller pumps the monitoring may be infrequent, say every three years, but the larger plants, having pumps greater than 150kW, would benefit from permanent monitoring. It is only by comparing plants, you see where you may be able to improve. The data for the water operations has been normalised to 1,000 population/m head. This allows all the companies, irrespective of topography, to be compared with one another.
All the companies have similar problems and various advantages and disadvantages but how is it that one water company's pumping cost is 65% greater than the average? In this age of accountants and quantity surveyors, when the price of everything is known but value is unknown, it is senior management that needs to take action because few engineers are allowed to operate as freely as their professional expertise should allow.WASTEWATER works
The situation at WwTWs is further complicated by the fluid being treated. On the pumping side, we have the situation where the requirement to pump solids usually precludes the use of the most efficient pumps. Also, the requirement to handle a far less predictable flow than in a water system makes optimal design difficult. Even so, good engineering practices should prevail and an economic compromise between efficiency and pumping constraints should be reached.
Starting firstly with the typical sewage sump. Here we have the most inefficient method of pumping one can imagine. For instance, to raise the fluid a few metres the first thing we do is to drop it 5m or so into a sump so no matter how efficient the pump is it is almost impossible to achieve an overall pumping efficiency of even approaching 50%.
The sump will then operate between two levels and except for a few systems there is no anticipation of pumping requirements. For instance, if the pump has just stopped pumping and a storm starts, the pump will not run until the top level has been reached and valuable pumping time has been lost. The pumping cycle itself leaves a lot to be desired, for instance, the operators like to drain the sump as low as possible, ignoring the fact that for several minutes the pump may be doing little more than trying to pump air.
This situation is further complicated by the size of some of the pumps. One installation has dry weather flow pumps of 500kW and storm pumps of 1,000kW. These pumps are not suited to sump emptying duties. In this instance a jockey pump should be used. Modern level detection and digital control systems should allow good control of the sump and along with the use of well-designed sumps and small jockey pumps, significant energy savings should be made. Sewage pumps have a greater wear rate than water pumps and it is imperative if pumping costs are to be kept within reason that the pumps are maintained within 10% of their optimum performance. Sewage pumping systems should be check-ed at least annually and some permanent monitoring systems considered.Blowers
Modern WwTWs use vast quantities of air, which is extremely expensive to blow into the system. Blowers and their associated systems should be checked on an annual basis and the following items considered:
- filters - dirty or blocked filters can increase the energy consumption by as much as 20%,
- leaks - you can usually see a water leak but it is not so easy with air,
- air usage - only use the amount of air that is required for the process. Excessive use of air raises electricity costs.
One example is that a WwTW had to reduce its ammonia level from 40-10ppm so a new BAF plant was installed. Great, the ammonia level was now down to 1ppm. The cost of this operation in electricity terms was in the region of £60,000 per year, half of which could be saved if the ammonia level was controlled to the required level. In reducing energy usage in water and WwTW there are several general points to consider. These include:
- testing pumps on a regular basis - ensure they are refurbished at the optimum time,
- reduce head losses in pumping systems - this is particularly important on the suction side of the pump,
- only pump the required amount,
- set up monitoring and targeting systems,
- review each station's performance on a monthly basis,
- compare the performance of different installations - ignore excuses for poor performance.
Above all, enjoy yourselves, it not only saving your company money but also reduces CO2 emissions and so helps to improve our environment