Pumps and Compressors for the World Market with Compressed Air and Vacuum Technology 2012

Reciprocating compressors in use in photovoltaics and for biogas feed

Special strengths in the area of sustainable, resource-saving energies

Martina Frenz and Dr. Mike Hüllenkremer

To date, reciprocating compressors have mainly been used for the transport and further

processing of fossil raw materials and for manufacturing their derivates – key industries

which made no visible contribution to environmental protection. However, since the

range of applications for compressors was extended by regenerative energies during

recent years and contribute to resource-saving energy production, they occupy the

field of sustainable technologies in machinery and plant manufacture positively. Their

depiction in photovoltaics and in biogas feed proves all the more that they have long

since occupied their permanent place for the provision and use of environmentally

friendly energies. However, they can only contest this importance by using innovative

techniques.

Reciprocating compressors occupy the field of sustainable technologies in machinery and plant manufacture positively.Photo: Fotolia / Franz Metelec

Reciprocating compressors had already always

played a pioneering role when implementing

demanding market requirements, the same also

applies to low emissions. They have met this

postulate for several decades in both the oil and

chemicals industries due to the compression of

H2 and H2 gaseous mixtures in systems for hy-

drodesulphurisation (HDS) and recycling flare

gas in refineries.

The German Federal Government’s energy con-

cept to reduce CO2 emissions by 40 % lower

than 1990 levels by 2020 resulted in support

measures which stimulated the renewable

65COMPRESSORS, COMPRESSED AIR & VACUUM TECHNOLOGY

Pumps and Compressors for the World Market with Compressed Air and Vacuum Technology 2012

energies market and primarily the expansion of

solar installations. Photovoltaics development

has gained considerable self-momentum, in

particular in the field of electricity where recip-

rocating compressors play an important role.

Photovoltaics – mainstream of the future

According to the Bundesverband Solar [Federal

Association of the German Solar Industry], so-

lar electricity’s share of German electricity re-

quirements will rise from currently 2 % to about

10 % by 2020. According to the forecast by the

Verband Deutscher Maschinen- und Anlagen-

bau (VDMA) [German Engineering Federation],

European electricity generation will be covered

roughly fifty-fifty by conventional and renew-

able energies by 2030. Taking 2007 as a basis

it is expected that the share of regenerative

energy in European electricity production will

even have tripled by then from 16 to 48 %. So-

lar electricity will therefore develop to become

a mainstay of the sustainable energy system.

From a global view, China, the USA, and South-

ern Europe are the future growth drivers in this

market segment; to a significant extent now

Korea, Taiwan and India, too. Analysts expect

growth rates of up to 130 % per annum on these

markets.

For manufacturing purest polysilicon, the ini-

tial substance for photovoltaics systems, both

smaller and larger compressor series with an ac-

tuating power which varies between 25 kW and

in excess of 3,000 kW are necessary. Since then,

the vertical dry running type convinces world-

wide with its space savings and higher durabil-

ity. Machines with higher construction scales

dominate supplies increasingly during the past

two years. This is not really surprising because

the global market is hard-fought and manufac-

turers can only survive through “economy of

scale”. In the Middle Kingdom, the government

actually prohibits the construction of small sili-

con factories. The consequence is “Monster fac-

tories in the gigawatts range” which integrate

the upstream and downstream processes in the

value-added chain. However, for what process

steps in the manufacturing of polysilicon does

the reciprocating compressor make a positive

contribution to the preservation of resources?

In order to understand this, the manufacturing

process for purest silicon is outlined here briefly.

However, the further process steps of pulling

single crystals as a basic material for producing

“wafers” which are used as solar cells in photo-

voltaics systems are done without here as they

are irrelevant for the compression process.

The production of purest silicon

The initial substance for the extraction of purest

silicon which is used to produce microchips and

solar cells is metallic raw silicon, a very hard and

brittle metal which is approximately 99 % pure.

The raw silicon is chemically cleaned in a highly

technical process. For this purpose, the raw sili-

con metal is first ground and a reaction is start-

ed in a reactor with hydrogen chloride (HCl). The

raw silicon and gaseous hydrogen chloride react

Figure 1: The application of reciprocating compressors in the polysilicon process for the preparation of gas mix for the ensuing cleaning

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Figure 2: Series of vertical, two-crank and dry running hydrogen compressors size 130 with a 7,300 Nm3/h supply quantity for the polysilicon process

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chemically and convert to trichlorsilane (TCS),

a completely clear liquid. In a further step, TCS

reacts with pure H2 in the reactor. The result is

pure silicon as a polycrystalline and a gas mix

consisting mostly of H2 as well as a residue

with HCl and the three chlorsilane compounds

dichlorosilane, trichlorosilane and silicon tetra-

chloride (STC). The gas mix runs through several

“cooling cascades” in order to dehydrate compo-

nents. If not all the components are separated

from one another after the first run, the pres-

sure is increased and further cooling takes place.

The aim is to collect the exhaust gas from the

reactor and separate its components fully

through condensation. The thus recovered

hydro gen is fed to the CVD reactor again for

crystal formation. The hydrogen chloride is used

to manufacture TCS. The released STC is then

converted to TCS, the initial substance for crystal

formation, in a further process step.

Figure 3: Area of application for oil-free reciprocating compressors for producing biogas and for the compression of biomethane for pipeline network feed or feeding back into the network.

Figure 4: Special shaft seal on the crankcase for guaranteeing external emission freedom

Reciprocating compressors are used in this

process in order to prepare the gas mix arising

from the reaction for the following cleaning by

increasing the pressure. In the gas preparation

system, firstly the chlorsilane residue is elimi-

nated and then hydrogen and hydrogen chloride

separated so that it can then again be fed into

the cycle. The sustainable contribution by recip-

rocating compressors is therefore applied in two

senses during the partial process of gas develop-

ment: on the one hand through energy-saving

as the developed gases no longer need be gen-

erated expending energy and on the other hand

by the hindrance of letting off the gases into the

earth’s atmosphere.

Biogas for an environmentally friendly future

As a renewable energy carrier, biogas is a multi-

talent. It is used for power generation, heat re-

covery and as a fuel in the area of mobility. At

the same time, biogas can be extracted from

various raw materials and residual materials.

In particular processes which use agricultural

remnants such as liquid manure, manure, or-

ganic waste and straw make biogas a renewable

energy source without using foodstuffs.

Biogas can be technically cleaned to natural gas

quality and fed into the existing European natu-

ral gas grid because natural gas and biogas have

an identical chemical composition.

For this purpose, the biomass remains some 30

days in a large fermenter where bacteria are

added. During fermentation, raw biogas is pro-

duced, consisting some 60 % of methane. The

rest is carbon dioxide, sulphur and anorganic

elements. They are cleaned in further steps by

a desulphurisation and CO2 separation system.

The purified biomethane can then be fed into

the natural gas grid. As a fuel, it is available at

natural gas filling stations, the difference being

that as opposed to petrol it reduced CO2 emis-

sions by 90 %. This makes biogas plants an im-

portant milestone on a genuinely sustainable

route. This all the more if the Federal Govern-

ment’s ambitious targets are realised, i. e. if

6 billion m3 biogas are fed into the public gas

grid by 2020. This could secure 6 % of the natu-

ral gas consumption. In this scenario, one of the

beacons of hope is the currently largest biogas

plant in Schwedt in the Uckermark region in

Germany which already yielded an output of

30 MW in its first expansion phase in 2010 and

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which strives to double this by 2015. But what

requirements must the reciprocating compres-

sor meet for this application?

Technical requirements for the biogas

compressor

The postulate of the Biogas Association to use

low-leakage compressors is already known to

quite some compressor manufacturers since

the 1980s when back then customers from the

chemicals industry already required machines

with special crankcase sealing for ethylene com-

pression. This know-how was requested again

20 years later when inquiries from the area of

“biogas feed” involved testing the crankcase for

pressure resistance. If using drive mechanisms

for the process gas industry, which per se have

a robust design, reinforcements of these drive

mechanisms were only necessary on the flanges

and cylinder covers according to FEM calcula-

tions. The following test run with gas admission

confirmed that only considerably less gas than

the required limit escapes from the suitable V-

type drive mechanism at a rotational speed of

1,500 min–1. Whilst some compressor concepts

on the market let the gas escape into the atmos-

phere, solutions which are designed for emis-

sion-free operation from the start have a con-

siderable advantage. The suction side and pres-

sure side single block fittings are only closed for

servicing and the residual gas in the compressor

is discharged into the atmosphere.

The range of services for each process

With the development of gas-tight drive mech-

anisms, compressors with the compact V-type

construction can operate all biogas generation,

biogas treatment and biomethane use pro-

cesses. After cleaning and adaptation of the

calorific value of the gas, the compressor takes

over either the feed of the biomethane into the

natural gas pipeline network at up to 90 bar

discharge or feeding back the excess natural

gas into the transport network. Compression

should always be made oil-free, i. e. without oil

Photovoltaics development has gained

considerable self-momentum, in particular

in the field of electricity where reciprocating

compressors play an important role.

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The SIGMA AIR MANAGER maximises compressed air system efficiency by:

Maximum Efficiency•eliminatinginefficientidlingperiods•minimisingcontrolandswitchinglosses•reducingsystempressure throughanticipatoryswitching

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Switching losses Control losses

INNOVATIONS & TRENDS68

Pumps and Compressors for the World Market with Compressed Air and Vacuum Technology 2012

contamination in the cylinder chamber in order

to fulfil the gas quality according to DVGW G

260. The drive mechanism must be designed to

be gas-tight and pressure-resistant, so that no

leakages are incurred. The compressor only then

guarantees emission-free compression under

realistic operating conditions. The gas-tight de-

sign of the V-type drive mechanisms is available

dependent on suction pressures from 1.5 to 10

bar in a one- to four-stage version and for supply

quantities between 350 and 3,500 Nm3/h.

Scope of delivery with a view

to the surroundings

The biogas compressor is normally placed rigidly

on an even bottom plate which must be decou-

pled from the building with regard to the trans-

mission of vibrations. For “turnkey” compressor

systems erected on a stable base frame, the

system components are anchored firmly on the

frame, enabling smooth running when operat-

ing. For operators who also involve the aspect of

environmental protection, erection can be made

in a concrete container – which can be installed

turnkey – on a foundation. They protect the

environment against noise and unobtrusively

adapt to their surroundings. Service concepts

with individually tailored servicing contracts

which also include the setting up of a consign-

ment warehouse for wear and spare parts are

options which ensure high availability of the

compressor system, placing less burden on en-

ergy providers and plant manufacturers.

Standards and regulations

The following implementation rules must be

met by compressor systems for network feed:

• DVGW work sheets G 498

and G 497 – VP 265-1

• HL-VO/DIN 30690

• D GRL 97/23/EC

• Ex (DVGW-HWG 442)

• ATEX Guideline 94/9 EC

• Ex zone 1

Biogas compressors which moreover fulfil the

individual feed standards from transport com-

panies and energy providers ensure themselves

a dominating position for this application.

Energy efficiency concerns everybody

In addition to the “renaissance” of the renew-

able energies, the reduction of energy con-

sumption also plays an important role. It is still

viewed as the most efficient method to obtain

strategic competitive advantages. Against this

backdrop, innovative techniques for reciprocat-

ing compressors which increase energy effi-

ciency are sought for such as those for output-

adjusted regulation of the volume flow. Of all

applications, the feed-in and withdrawal of nat-

ural gas in underground caverns poses the high-

est demands on the flexibility of large volume

flows. Therefore, the major target was to align

research and development work with this target

In addition to the “renaissance” of the

renewable energies, the reduction of energy

consumption plays an important role.

Figure 5: High-performance compressor size 130 for bio-methane feed on a base frame with supply quantities up to 3,500 Nm3/h and power exceeding 500 kW

Figure 6: Oil-free biogas compressor system size V1 in a container for effective mobility and protection against noise emission

69COMPRESSORS, COMPRESSED AIR & VACUUM TECHNOLOGY

application and to assess the knowledge gained

based on real conditions. The evaluation process

for various concepts and the newly developed

alternative solution for efficient, stepless vol-

ume flow control are briefly explained below.

Variable stroke concepts for

loss-free volume flow regulation

The evaluation phase for standard concepts

starts with literature, market and patent re-

search. The most common volume flow princi-

ples such as speed regulation, cylinder clearance

change and suction valve unloading in their

stepless and cyclical version were identified and

evaluated. They all contain advantages and dis-

advantages which are long since known to both

the compressor manufacturers and operators.

Although speed control was top of the ranking

in the volume flow concepts with reference to

energetic assessment, the actual disadvantage

of vibrational excitation in the natural frequency

range excludes certain speed ranges from use.

Consequently, speed controlled machines can

only be used with restrictions. An initial solu-

tion of additional clearance pocket provided an

interesting alternative from a thermodynamic

view, however it was discarded due to its lower

efficiency. Variable stroke control, which can be

placed between the two previous concepts, was

identified as a highly promising alternative solu-

tion. A control mechanism which promises low

energy loss and stepless volume control which

has not yet achieved market maturity.

The question of the feasibility of such a mecha-

nism is posed below. For this purpose, various

approaches were viewed, assessed and devel-

oped until the solution of a six-bar linkage of

the crank drive as a replacement for the com-

mon slider crank was seen to be effective. There-

fore, it was necessary to divide the connecting

rod in two parts which are connected with a

joint. An additionally attached rocker restores

the crosshead’s automatic run. It is connected

Your contact person: Mark Decker, Tel. +49 511 89-31127, mark.decker@messe.de

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NEW TECHNOLOGY FIRST8–12 April 2013 · Hannover · Germany

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jointed to both connecting rods and a guide as-

sembled in the crankcase, thus compensating

the additional degree of freedom. The stroke is

influenced by the position of the rocker during

its displacement and therefore the volume flow

is controlled.

The advantages of this innovative variable stroke

control are clear: in addition to continuous con-

trollability, this concept provides an energeti-

cally highly advantageous solution against its

“colleagues”. Thus, for example, it enables con-

trol between 40 and 100 % of the volume flow

with a variable stroke of 41 mm of a four-crank

natural gas compressor with a nominal stroke

of 300 mm! As for the familiar cyclical suction

valve unloading, hydraulic drive is also required;

however the difference is that it assumes the

positioning of the rocker along its guide track.

However, the drive is exclusively activated for

the readjustment of the volume flow with the

variable stroke. A further inherent system ad-

vantage is lower pulsations on the suction

side which therefore means that the pulsation

damper may be kept smaller. ATEX-conformant

versions are possible.

In February 2011, loss-free volume flow control

through variable stroke control was registered

as a patent – an innovation which is competi-

tive both from a price and a technical viewpoint.

However, the potential for innovation is by no

means exhausted. A second, promising solution

for stepless variable volume flow regulation is

currently being worked on and will soon be ma-

ture for the market. Reciprocating compressor

operators can then manage optimum energy

management in their plants – right in the sense

of Blue Competence, the sustainability initiative

of the German mechanical engineering industry.

Authors:

Martina Frenz

Dr. Mike Hüllenkremer

NEUMAN & ESSER GmbH & Co. KG,

Übach-Palenberg, Germany

Figure 7: Concepts for volume flow regulation. Comparison of degrees of efficiency relating to effective crankcase performance. Only the solution of variable stroke control meets both requirements of energy efficiency and stepless volume flow control.

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Figure 8: Comparison of variable stroke control and synchronised suction valve unloading with regard to the relative work performed applied to the relative volume flow. Data was established for common process gases on pV graphs.

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