Energy efficiency in stationary hydraulic systems

Energy policy in Germany and worldwide has become increasingly important in recent decades. One of the aims of Germany’s energy revolution is to cut carbon dioxide emissions, and an important aspect of this is to reduce energy demand by increasing energy efficiency. The pie chart in Fig. 1 shows that the increase in efficiency in the industrial sector is having a major influence on energy consumption in Germany. According to this analysis, the industrial sector accounted for around 30% of Germany’s total final energy consumption in 2017.

In addition to the benefits to the environment and society as a whole, increasing energy efficiency is also important for operators of hydraulic systems. On the one hand, increasing the efficiency of hydraulic systems can boost productivity while maintaining the same level of power consumption. At the same time, an increase in energy efficiency can lead to lower power consumption. This makes it possible to reduce electricity consumption and thus operating costs.

In stationary hydraulics, in particular, operating costs often play no part or only a minor role when an installation is purchased. For the plant operator the acquisition costs are much clearer, because they are comparable and direct. However, in the long run this approach is not economical for the operator of a hydraulic system. On the contrary, in order to reach an effective investment decision, the total cost of ownership (TCO) should be analysed. When examining the TCO for a pump system, for example, energy costs can amount to up to 80% of the total cost (see Fig. 2). In this example, the acquisition expenditure only accounts for 10% of the total costs.

Due to the lack of a TCO approach on the part of the operator, the primary goal of the installation manufacturers is to offer hydraulic systems at a favourable purchase price. Although these systems may meet the operator’s requirements in terms of function, a more precise system analysis reveals them to be true energy guzzlers. As a rule, no separate power meters are installed for individual hydraulic units. This makes it difficult to determine the energy efficiency of a particular unit when several electricity consuming units or hydraulic systems are operated simultaneously. Nevertheless, the system operator can check by simple tests whether the hydraulic system offers the potential for increasing energy efficiency:

• Does the oil temperature in the tank often exceed 60°C?
• Are some valves in the hydraulic system significantly hotter than others?
• Are negative/pulling loads often moved (load direction operates in the direction of movement)?
• Are flow control valves (throttles, flow regulating valves, proportional valves) installed?
• Are several consuming units supplied simultaneously by one pump?

If the answer to one or more of these questions is “yes”, a system analysis may be worthwhile. It is not uncommon for an oversizing of volume flow or pressure – and in the worst case both – to be identified. An example of oversizing is the selection of a large pump with a maximum flow rate which is greater than what is actually needed. For a variable displacement pump, this means operation with a reduced swivel angle. As is indicated by Fig. 3, a total pumping efficiency of 85% can only be achieved with a swivel angle between 60% and 70%. If the swivel angle falls below 20%, the overall efficiency is less than 50%, so that the power loss is greater than the effective hydraulic power. A further example is a constant flow system with a needs-based volume flow reduction by throttling. The excess volume flow is relieved at the tank by a pressure relief valve. The energy required for the relief is converted fully into power loss. As a rule, the replacement of a single valve does not lead to an increase in energy efficiency, and instead the hydraulic system has to be completely overhauled.

The hydraulic system of the test laboratory at Bertrandt Ingenieurbüro GmbH Munich provides an example for the analysis of energy efficiency. The central hydraulic unit has a connected load of approx. 130 kW. A variable displacement pump supplies several test stations with a constant pressure of approx. 260 bar. The volume flow rate depends on the selected test stations and test programmes. The test stations all consist of distributor blocks with downstream servo cylinders. Operation of these test stations shows that at a system pressure of approx. 260 bar and a volume flow requirement of approx. 90 l/min, the hydraulic output is 37 kW (see Fig. 4). In addition, the minimum system pressure was determined by manually adjusting a pilot valve. The test stations could also be operated with a system pressure of 150 bar without any problems. By reducing system pressure it proved possible to reduce the hydraulic power to 22 kW. This results in a 40% increase in energy efficiency. The energy saved for this operating point in the partial load range is 120,000 kWh per year. Since not all test stations were operated during the analysis, the overall savings potential is in fact even greater. In addition to energy savings, the thermal load on the oil and the pressure load on all the components in the system are also reduced.

The International Hydraulics Academy (IHA) will be happy to advise you on questions concerning the energy efficiency of your hydraulic systems. The consultation includes an individual system analysis as well as the development of solution proposals for a retrofit. The Federal Ministry of Economics and Energy subsidises up to 40% of the costs of measures to save energy and reduce carbon dioxide emissions (see KfW Reconstruction Loan Corporation Credit 295 Module 4: energy-related optimisation of installations and processes).