oj_Artikel.pdf

Development and Field Test of a SOLO 161 Stirling Engine based

Micro-CHP unit with Ultra-Low Emissions

Magnus Pålsson

Dep’t of Heat & Power Engineering, Lund University

P.O. Box 118, S-221 00 Lund, Sweden

Magnus.Palsson@vok.lth.se

http://burn.at/stirling

Abstract

For the last decade, work has been made at Lund University, Sweden, on developing a new sort of

natural gas combustion chamber for the V160/SOLO161 Stirling. It is a lean premix combustion

chamber with internal combustion gas recirculation and a metallic flame holder for flame stabilisation,

and it has produced extremely low emissions that are comparable to the emissions of catalytic

combustion. The combustion chamber is considered ready for market introduction by the end of 2001.

To combine the task of adapting this combustor for the market with the need to demonstrate small-

scale Stirling engine CHP technology, a project has been started with the purpose to demonstrate and

evaluate the operation of a Stirling engine unit based on the SOLO 161 Stirling engine equipped with

the Lund combustion chamber. The evaluation program should give information regarding operation

costs, efficiencies, emissions and running characteristics.

In November 2000 the engine was transferred from the Lund University laboratory to its final location

in Gothenburg. The engine is now installed and all necessary adaptation of engine, gas system and

water heating system is made. The unit is running unattended in normal everyday operation. Current

operating time is approx. 1200 hours, and delivered electric output is approx. 6000 kWh (July 2001).

Introduction

Decentralised combined heat and power (CHP) is an area that gets more and more attention. Gas

engine-based decentralised CHP have been installed in sizes of approximately 0.5 to 10MW. Other

sizes occur, both bigger and smaller, but the main part of the units installed is within this power range.

As further experience of installing and running these units is made, the interest in making use of

smaller heat demands as well, e.g. schools, apartment blocks, sports centres and even private houses,

is increasing.

The Stirling engine technique has been developed over several years and today there are a number of

different models and sizes. What they all have in common is that they are in a phase of development.

Thus they have not been demonstrated in a greater extent as small-scale power heating units. A brief

summary of the present situation with regard to the Stirling engine is given by the Swedish Gas Center

in [Ref. 3]. To this should be added the work that Gasunie does to adapt a Whispertech 800 W Stirling

engine unit to the market for use in small houses as a CHP unit, and also Ocean Power

Corporation/Sigma Elektroteknisk’s plan of developing and introducing a 3kW unit to the market.

At Lund University, Sweden, the Stirling engine competence is solid and work is constantly made on

developing combustion systems for Stirling engines. During the last decade extensive research has

been made on a lean premix prevaporized combustion concept with recirculation of the inert burnt gas

and a metallic flame holder. From this concept a new lean premixed natural gas combustion chamber

with internal combustion gas recirculation (CGR) has been developed for the V160/SOLO 161 Stirling

engines. This combustor has ultra-low emission levels, comparable to those of catalytic combustion.

At the start of the current project the combustor was ready to be adapted for production, with expected

market introduction in 2001. In Table 1 below the emissions from the old V160 swirling diffusion

combustion chamber are compared with the design goals for the new combustion system.

“Old” V160 combustor “New” SOLO161 combustor (design goal)

HC < 1 ppm < 1 ppm

NO x 220 – 650 ppm < 10 ppm

CO 150 – 800 ppm 30 – 50 ppm

Table 1 – Emissions (design goal) for the new combustor compared with the old combustor.

Project goal

The purpose of this project is to demonstrate and evaluate the operation of a Stirling engine unit based

on the V160/SOLO 161 engines equipped with a combustion chamber with extremely low emissions

as described in the last paragraph.

The project can be divided into three different tasks:

• Selection of field test location and of operation strategy

• Installation in laboratory and laboratory testing

• Running/demonstration/evaluation of the CHP-unit in a commercial installation

Selection of field test location and of operation strategy

The unit had to be placed in an environment, which on one hand suits the engine power level and on

the other hand suits the unique running qualities that the unit possesses, i.e. low sound level, minimal

vibrations, low emissions.

Supply (forward) temperature

Secondary

hot wa ter circ uit

Natura l Gas

Furnace

ADDED

Return

temperature

Ambient

outdoor

temperature

Stirling E ngine

Figure 1 – Secondary hot-water circuit after installation of Stirling engine unit

A suitable location was established together with Göteborg Energi, the local utility company in

Göteborg, Sweden. The engine was placed in a small boiler station that supplies heat to a combined

office, workshop and warehouse in Marieholm, Göteborg. Furthermore, the building belongs to the

utility company. The supplied building has a total area of 2500 m 3 and the heat is supplied from a 250

kW hot water boiler. The yearly energy demand is 284 MWh and a SOLO 161 engine with an output

of 20 kW heat can cover approximately half of that heat demand.

A hot-water scheme like in Figure 1 was decided on, and the supply and return temperatures for this

scheme are shown in Figure 2. It was decided the Stirling engine unit was to run constantly at 12MPa

cycle pressure during the winter season when the heating demand is great, and run in on-off mode in

the summer season when the heat demand is less than the units heat output at 12MPa continuous

operation.

Supply temp

Return temp, day

Return temp, night

−20

−15

−10

−5

Outdoor temperature [ ° C]

Figure 2 – Secondary hot-water circuit temperatures

Installation in laboratory and laboratory testing

SOLO Kleinmotoren and Intersol have supplied a SOLO161 Stirling engine unit to the project. The

unit was first installed in the Lund University (LU) laboratories. LU together with Intersol have

designed and installed the combustion and heater system for the engine. The combustion system is

based on a LPP combustion system that has been developed at LU [Ref. 1, 2]. The engine was

provided with equipment necessary for the supervision and computer communication of the project.

Initial tests along different development lines were made on the Lund University United Stirling

V160F laboratory engine, which was equipped with an experimental burner (Figure 3). At this stage

tests concerning general combustor layout were made, including tests with different flame holders,

with varied mixing tube length, with varied number of air nozzles, with varied air nozzle diameter and

with different methods of natural gas injection [Ref. 2]. Also, tests aimed at lowering system

cooled

stirling

engine

heater

preheater

combustion gases

exhaust gases

CGR

flame

holder

inlet

air

pre-

heated

flow guide

body

fuel

mixing tube

air

hot combustion

gases

Figure 3 – Schematic view of the Lund V160 experimental combustion chamber

Figure 4 – The new combustion chamber mounted on the SOLO 161 unit.

pressure losses were made, including tests with varied lambda and combustion gas recirculation

(CGR) rates, so that the standard SOLO161 air blower could be used, and tests aiming at adapting the

burner from the lab 3.6 bar natural gas grid absolute pressure to a natural gas pressure of 0.1 bar. The

evolution of a suitable start sequence was initiated. When the general combustor design was decided

on, a prototype combustor was designed and manufactured for the SOLO 161 engine by Intersol. The

SOLO 161 engine and control system hardware (Figure 4) was adapted to the new prototype

combustor, which was fitted to the engine for further tests (Figure 4). The combustion system tests led

to the decision to re-design the flame holder and its internal support. However, this was the only

change made from the original prototype design.

Thermocouple

Helium temperature

Thermocouple

Mixtube temperature

Pressure transducer

Cycle pressure

Pressure transducer

Bottle pressure

RPM co unte r

Oil pressure guard

Coolant temperature

guard

Electronic control unit

Supply valve

Helium

Dump valve

Helium

Air blower

Ig ni ti o n co i l

Main switch

generator

Primary main

NG va lve

Secondary

main NG

val ve

Choke valve

Spark plug

380 V

50 Hz

NG p ressure

regulator

Combustion

chamber

Figure 5 – Engine and combustion control hardware - schematic

Electr(on)ic

Air

SOLO 161, Lund combustor, p Cycle = 120 bar (001103 1 )

NO x [ppm]

C 3 H 8 [ppm]

CO/100 [ppm]

1.2

1.3

1.4

1.5

1.6

1.7

1.8

(calculated from exhaust composition)

Figure 6 - Combustor emissions for the unit at full load, measured just prior to delivery to Göteborg.

A suitable control strategy had to be determined, along with the selection of an “exact enough” fuel

control valve. In the end a Kromschröder air/gas ratio control was chosen, that regulates inlet gas

pressure to the same pressure as that of the inlet air (c.f. Figure 5, NG pressure regulator). An on-off

gas valve (c.f. Figure 5, Choke valve) in series with a restricting orifice was mounted on a parallel fuel

line for cold start gas flow control. Final cold and hot start-up control sequences were decided on and

programmed to the electronic control unit’s EPROM memory, see also [Ref. 2].

At the end of the laboratory test period the engine was mechanically inspected and the PL seals were

renewed before it was transported to the commercial site in Göteborg, in order to ascertain that the

engine was in an “as-good-as-new” condition at delivery. Also, emission tests at full load for different

lambda values were made for comparison with later field test results (Figure 6). The development and

testing of the combustion system is described further in [Ref. 2], presented at this conference.

Figure 7 – Installation of the SOLO161 unit in a small boiler station in Marieholm, Göteborg.

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