E. Uhlmanna, L. Prasola, S. Thoma, S. Saleina, R. Wieseb
aInstitute for Machine Tools and Factory Management IWF, Technische Universität Berlin,
Pascalstr. 8-9, 10587 Berlin, Germany
bBeuth University of Applied Science, Luxemburger Straße 10, 13353 Berlin, Germany
Abstract. The growing awareness for sustainable production and increasing energy
costs as well as the commitment to legal restrictions lead to a rising demand for energy efficient solutions for long-life production facilities. The applicability of energy harvesting concepts, in order to increase the energy efficiency of highly dynamic machine tools with linear direct drives, is one research objective at the Institute for Machine Tools and Factory Management (IWF). Therefore, thermoelectric generators are placed in the heat flow between the primary part of a linear direct drive and the cooling system to convert parts of the thermal losses into electric energy. It was investigated, if a so called thermoelectric self-cooling-system is applicable to operate a pump of a water cooling circuit, only supplied by thermoelectric generators. To determine the harvested energy and to simulate the steady and the transient state of the system a thermoelectric model was developed. The comparison of simulative and experimental determined data indicates high model prediction accuracy. Hence, the model turns out to be a powerful tool for the development and analysis of water flow thermoelectric self-cooling-systems. The results show that the thermoelectric generators can provide sufficient energy to operate a water pump in a decentralized cooling system with reasonable coolant flow rates.
Peer-review under responsibility of the International Scientific Committee in the person of the Conference Chair Prof. Steffen Ihlenfeldt.
Keywords: Machine tool; Linear direct drives; Thermoelectric self-cooling; Energy efficiency
1 Introduction
In 2015 the industrial sector accounts for about 42 % of the global energy con- sumption [1]. A substantial proportion of this demand is used for the operation of machine tools [2]. As a result of a growing awareness towards energy saving solutions, the European Commission has passed a series of directives [3,4,5].
List of symbols Subscripts
A Area (m2) ab Absorbed
C Thermal capacity (J/K) c Cold side, outer surface ceramic
IL Electrical load current (A) cd Heat conduction
L Length (m) c,eff Cold ends of the semiconductor
legs
m Mass (kg) cer Ceramic
Nu Nusselt number cont Thermal contact
PC Cooling capacity (W) cp Cold plate
Pel Electric power generated by TEG (W) cs Cooling system
Qሶ Heat flow rate (W) cv Heat convection
R Thermal resistance (K/W) dev Device to be cooled
Ri Electrical internal resistance of TEG (Ω) exp Experimental value
RL Electrical load resistance (Ω) ge Generated
T Temperature at a particular time t (K) h Hot side, outer surface ceramic
t Time (s) h,eff Hot ends of the semiconductor legs
UL Load voltage (V) hs Heat source
UOC Open-circuit voltage (V) in Inlet coolant
Vሶ Volumetric flow rate Vሶ (m3/h) ind Induced
Greek letters J Joule effect
α
ഥ Seebeck coefficient (V/K) out Outlet coolant
ο Time step (s) P Peltier effect
λ Thermal conductivity (W/mK) res Reservoir
sim Simulated value
Within the subject of sustainable manufacturing, machine tools are considered as energy relevant by the EU legislation and thus, there is a need to follow eco- design measures as defined by directive 2009/125/EC [3]. In machine tool life cycle the energy consumption in the operation phase was identified as the dom- inant contributor [6]. Besides optimizing the energy efficiency and to meet the legal restrictions, the influence of thermal losses on the thermal behavior of ma- chine tools must be considered. Inefficient energy transformation in machine tools induces heat flows and leads to an increasing temperature of precision re- lated machine tool components. This change in temperature causes a thermo- elastic deformation of the machine tool structure, which directly influences the position of the tool center point, and therefore the working accuracy of machine tools [7]. In order to reduce the influence of heat sources on the accuracy of ma- chine tools, both the development of energy efficient components with limited heat input and active cooling of relevant components are possible solutions [7]. Cooling systems are capable of ensuring an nearly uniform temperature distribu- tion along the precision related components and therefore reduce the overall thermo-elastic deformations of the machine tool structure [7]. Energy related research reveals that auxiliary units for the supply and conditioning of fluidic me- dia account for a major share of total energy consumption [8]. This applies espe- cially for machine tools which are subjected to high requirements regarding productivity and accuracy. In high dynamic machine tools for productive manu- facturing of high-precision parts linear direct drives (LDD) are increasingly ap- plied to allow higher acceleration, feed rate, and thus rapid positioning compared
to conventional ball screw drives [9]. In relation to the motion profile of a ma- chine tool axis the primary part of the LDD is responsible for significant thermal losses. The arrangement of the LDD next to precision related machine tool com- ponents requires a tempering to dissipate these thermal losses and to avoid high temperature gradients in the adjacent machine tool components [10].
SHABI ET. AL. [7] reveal that the cooling system of a machine tool with LDD of type
DMU80 eVo linear, DMG Mori AG, Bielefeld, Germany, requires 26 % of the total energy consumption. Additionally, 25 % of the total energy consumption is sup- plied to further consumers of the fluidic system. Based on these results,
SHABIET. AL. [7] states a major potential to reduce the energy demand and to in-
crease the efficiency of machine tools by improving the function of the fluid sys- tems [7]. Therefore, decreasing the base load by implementation of a demand based fluid supply is a promising approach [8]. This procedure is intended to avoid oversupply of cooling fluid regarding pressure, amount and duration [8]. To achieve the objective of reducing the energy consumption of machine tools, ex- tensive research efforts have been spent to identify, monitor, and visualize the main consumers as well as to develop shutdown control strategies for compo- nents during waiting time [10].
Moreover, there are detailed investigations about fluidic systems in machine tools to derive new concepts for the structure of the cooling system [11]. Based
on experimental investigations SHABIETAL. [11] show that sufficient cooling ca-
pacity is available but the cooling is insufficiently adjusted to the process and to the individual demand of the machine tool components. Based on a developed model of a decentralized supply-unit a demand-oriented cooling system has been studied. Therefore, it is proposed to provide the required cooling capacity for each component by controlling the coolant flow rate instead of the coolant tem- perature. This leads to a decoupling of thermal load and outlet temperature, and thus to maintain a stable outlet temperature. Due to the demand-orientated sup- ply of cooling capacity, the control of volume flow rate is a promising approach to reduce the required total pump power [11].