18.2.1 Rankine and organic Rankine cycle machines

In the Rankine cycle, water is converted into the relatively high pressure and temperature water vapour (steam) in a special high temperature heat exchanger (boiler/steam generator). This vapour is then used to expand and drive a prime mover. After the expansion process, vapour may be partially converted into the liquid and at the end of expansion this relatively low pressure and temperature liquid-vapour mixture is circulated through a low temperature heat exchanger (condenser) in which all vapour is converted into the liquid state due to the cooling process. This liquid then is returned to the boiler/steam generator using a small feed pump. In organic Rankine cycles (ORC) special organic fluids with lower boiling temperatures are used instead of water. The thermo-physical properties of such fluids allow a greater power output to be achieved for the same identical dimensions of the prime mover and of heat exchangers and the working fluid mass flow.

Prime movers are usually reciprocating piston (Rankine cycle), scroll or screw (organic Rankine cycle) mechanism machines. Figure 18.1 shows schematic of a double-acting piston arrangement in which steam or vapour is supplied in turn into cylinders on the left and right. When steam is directed into the left cylinder, this coincides with a discharge of vapour from the right cylinder. The pressure of steam drives pistons which are connected to a linear rotor of a linear alternator, used to convert kinetic energy of linear piston motions into electricity.

A scroll engine employs two identical involute spiral shape scrolls which are mated together inside the casing forming concentric spiral shapes (see (Fig. 18.2). During expansion, vapour formed in heat exchangers enters a casing through the inlet port in the centre of the casing and expands in crescent-shaped gas pockets forcing the orbiting scroll to orbit around the fixed scroll in the casing. The orbiting scroll is connected to the rotating shaft with a rotor of an alternator. During a single orbit, expansion of vapour takes place simultaneously in several gas pockets formed by two scrolls, providing smooth, continuous operation of the engine. After expansion the fluid/vapour mixture leaves the casing through the exhaust port.

The rotary screw engine layout is illustrated in Fig. 18.3. The engine consists of two intermeshing screws or rotors (also called male and female rotors, respectively) which trap gas between the rotors and the compressor case. Both rotors are encased in a housing provided with gas inlet and outlet ports. High pressure vapour enters the casing and expands forcing two meshed rotors into rotational motion. The male rotor is connected to a rotor of an alternator.

In both cases, engine lubricant is mixed with vapour and also used for cooling and sealing purposes.

18.2.2 Internal combustion engines

Internal combustion engines have been in commercial production since the second half of the nineteenth century and presently this is a mature and very well-established technology. The engines used for CHP are usually four-stroke machines with one or several cylinders depending on the power output required. The fuel is natural gas from mains, biofuels, biogas (formed in bio-digestion) or syngas (produced as a result of biomass gasification). In these engines a combustion chamber in each cylinder is formed by the top of the piston, the head of the cylinder with built-in inlet and exhaust valves, and a cylinder liner (see Fig. 18.4). In micro-CHP systems natural gas is usually used as fuel and engines deployed are a single cylinder spark ignition engine. The spark plug is also built in the head of the cylinder. In order to avoid overheating of components during the engine’s operation, there are water jackets in the cylinder and in its head. Engines used for CHP have, as a rule, a conventional crank-shaft drive mechanism, placed in the crank-case below the cylinder/s. In such a mechanism the piston is connected to the shaft using a rod and the reciprocating motion of the piston is converted into rotational motion of the shaft. Natural gas and air are mixed in the inlet manifold before entering the cylinder. The cycle starts when the piston is in its top position in the cylinder and can be split into four stages: induction stroke-air-gas in (see Fig. 18.4(a)); compression stroke – all valves are closed (see Fig. 18.4(b)); combustion and expansion stroke (see Fig. 18.4(c)); exhaust stroke-combustion products are forced out of the cylinder (see Fig. 18.4(d)).

18.2.3 Stirling engines

The Stirling engine is the common name for an external combustion heat engine which works on a closed regenerative cycle. Various design configurations have been developed for a wide range of applications which include solar dish/engine installations, biomass systems, air independent submarine propulsion systems, on-board power systems for deep-space missions, etc. This sub-section is focused only on Stirling engines developed for small- and micro-CHP purposes.

The operational principle of this engine remains the same as for any heat engine: the useful power output work generated during expansion of the working fluid should be greater than the work used to compress the working fluid during the working cycle. The major feature of the Stirling engine is that the working fluid is sealed inside the engine’s volume and heat is transferred to and from the working fluid and from and to the internal gas circuit, respectively, through specially designed heat exchangers, namely a heater and a cooler, which are parts of the engine’s design and maintained at constant high and low temperatures, respectively.

Figure 18.5 presents an example of a design arrangement of a Stirling engine. In this arrangement the engine has two variable working spaces, namely expansion and compression spaces, located in separate cylinders, namely ‘hot’ and ‘cold’ cylinders with their own pistons. These working spaces are connected to each other via channels of three special heat exchangers integrated into the design of the engine, namely the heater, the regenerator and the cooler. The walls of the ‘hot’ cylinder and the heater are kept at elevated temperature, using some heat source (combustion of fuels, nuclear heat, concentrated solar radiation, etc.). Similarly, the walls of the ‘cold’ cylinder and the cooler are kept at a lower temperature (e.g., using a cooling water jacket). The movements of the pistons are synchronised and arranged in such a way that the ‘hot’ piston leads by approximately 90-120 degrees of the crank-shaft angle. Such motions of the pistons result in the gas being located mainly in the ‘hot’ cylinder during the expansion stroke (heat is introduced into the cycle) and in the ‘cold’ cylinder during the compression stroke (heat is rejected from the cycle).

Heat exchangers for the introduction of heat (a heater) and its rejection (a cooler) should have a surface-to-volume ratio as large as possible. The regenerator, which is placed between the heater and the cooler, represents a porous medium enclosed in the metallic casing. This porous medium is made from a material with a high heat capacity and should ideally have an infinite radial and zero axial conductivities. It acts as a heat sponge: heat is transferred to the material of the regenerator and stored when the working fluid flows from the ‘hot’ to ‘cold’ zone. The stored heat then is returned to the working fluid when it flows in the opposite direction. Thermo-insulation is usually used to separate the porous medium from the walls of its casing in order to reduce heat losses.

The volumes of heat exchangers and of pipe joints are so-called ‘dead’ volumes since these decrease the compression ratio, which is the ratio of maximum and minimum volumes of the engine. In order to obtain high power output and efficiency, it is necessary to keep the compression ratio of the engine as high as possible. In Stirling engines the typical value of the compression ratio is about 2.

Figure 18.5 also shows examples of the variation of the volumes in the ‘hot’ and ‘cold’ cylinders, the gas pressure of such engines and their pressure-volume diagrams over a single cycle. The difference in areas of the pressure-volume diagrams for the ‘hot’ and ‘cold’ cylinders represents work produced in the cycle (the indicated cyclic work). The product of the indicated cyclic work and the number of cycles completed per second is called the indicated power of the machine.

The thermodynamic efficiency of the ideal Stirling cycle with an ideal regenerator is equal to the efficiency of the Carnot cycle. In practice, the effective efficiency of existing Stirling engines is somewhat close to, or even less than, that of spark ignition engines, although there is great potential for improvement.

Stirling engines can use any source of heat: fossil fuels (liquid, gaseous, coal), radioisotopes, burning of metals and renewable energy (hydrogen; biomass fuels in solid or gaseous state from wood, straw, etc., sewage; landfill gases; direct or accumulated solar radiation heat). When using fossil fuels, a continuous combustion process takes place in the burners and using different techniques it is possible to achieve a very low level of pollutants in flue gases. There is no explosive combustion of fuel in the cylinders and burning occurs at relatively low pressures. Furthermore, as there is no valve-train mechanism, the noise in an operating Stirling engine is significantly lower in comparison with internal combustion engines. When necessary, special drive mechanisms can be used to provide an almost ideal dynamical balancing of the engine, which virtually eliminates vibrations during its operation. The variation of the engine’s torque is significantly smoother than that in internal combustion engines.

The fraction of the heat rejected from the cycle into the cooling system of the engine is approximately double that in internal combustion engines. This heat then can be used for CHP purposes. Gases with a high heat capacity and low viscosity, such as hydrogen and helium, are used in machines with a high performance. The pressure inside the engine may be up to 220 bar or greater, hence special attention should be paid to the sealing to prevent the leakage of these gases. To restrict costs, engines may be designed to run using air, nitrogen and other easily available gases. However, this results in an increase in the dimensions of the engine for the given level of the maximum pressure in the cycle and power output.

Lubrication is another challenge when designing Stirling engines. It is undesirable to have an oil lubricant or its vapour inside cylinders and heat exchangers, since the lubricant may burn off and form a film deposit on the internal surfaces of the internal gas circuit of the engine. This will then dramatically decrease the heat transfer between the heat exchangers and the working fluid. An additional negative effect will be an increase in the hydraulic resistance of the regenerator, since it will be blocked by particulate matter in the products of the oil burning. Hence, in engines with oil lubrication of the drive mechanism, it is necessary to use special sealing techniques to prevent lubricating oil from passing to the cylinders. As far as the pistons are concerned, they have guiding and sealing rings made from fluoroplastic materials, similar to those used in compressors. When using piston rings made of this type of material, there is no need to specially lubricate them using conventional oils.

The ‘hot’ cylinder, the heater and the casing of the regenerator are made of a stainless steel type material in order to withstand high temperatures and pressures in the cycle (up to 1000 °C and 220 bar, respectively), which makes Stirling engines more expensive in their production than internal combustion engines.

Another challenge is in the complexity of the control system of Stirling engines. If necessary, the control of the machine is achieved by the combination of a reduction in the heat output from the combustion chamber, pressure in the cycle or alternatively by increasing the dead volume of the engine.

There are three main layouts of modern Stirling machines-so-called alpha, beta and gamma configurations. Fig. 18.6 shows a ‘V’ type alpha single-acting engine, which has two separate cylinders with pistons. Both the pistons are used to produce work from the engine. When using a conventional crank-drive mechanism the phase-angle in the motion of pistons is equal to the angle between the axes of cylinders, hence the cylinders are arranged at an angle of 90-120 degrees. An alpha engine can be designed to be a double-acting machine, in which the compression space of each cylinder is connected through the channels of the heat exchangers to the expansion space of the adjacent cylinder (see Fig. 18.7). Such configuration allows nearly double the power output for the same swept-volume of cylinders.

Figure 18.8 presents a general scheme of the beta-type Stirling engine in which two pistons are installed in a single cylinder. The top piston is not used to produce useful work and its only function is to control the flow of the working fluid from the compression to the expansion space and vice-versa and this piston is called a displacer. The useful work in the cycle is produced by the second piston which is called a power piston. Figure 18.8 shows one of the drive mechanisms used in such engine configurations, which is called a rhombic drive. This mechanism allows an ideal balance of dynamic forces to be achieved and takes the forces from the sides of the pistons. This results in reducing the wear of the sealing and the guiding piston rings. An ideal dynamic balancing also eliminates vibration during the operation of the engine.

Finally, Fig. 18.9 shows the gamma configuration engine in which the compression space is split into two parts. The first part is located in the ‘hot’ cylinder under the displacer and the second part is located above the power piston in the ‘cold’ cylinder. Both the parts are connected to each other by channels made in the casing of the cylinders or by pipes.

The design configurations of Stirling engines described above have kinematical drive mechanisms. Figure 18.10 demonstrates the general concept of a free-piston Stirling engine. In this concept the engine does not have a kinematical drive mechanism. The engine is usually of a beta or gamma type with a displacer and piston in a single cylinder and operates as a thermal auto-oscillating system. Pistons perform reciprocating motions, which are in fact forced oscillations caused by the varying pressure of the working fluid in the working spaces of the engine and the forces from mechanical or gas springs. The power piston is a ‘rotor’ of a linear alternator. Its reciprocating motion in the stator generates electrical power.

18.3.1 Rankine cycle micro combined heat and power (MCHP)

The Rankine cycle is based on external combustion and characterised by the working fluid undergoing a phase change (from liquid to gas) which can be utilised to achieve high power densities. The most familiar Rankine engine is the steam engine where water is boiled by an external heat source to cause an expansion and the generation of pressure which can be used to produce useful work.

MCHP production units implementing the Rankine cycle are entering the market and OTAG released the Lion Powerblock unit in Germany [1]. This is a double-acting free-piston steam engine with a linear alternator with outputs of 0.3-2 kW_el and 3-16 kW_th with overall efficiency of about 94%. The operating frequency is 40-75 Hz and noise level is between 48 and 54 dB. The dimensions of the MCHP is 126 mm (H) × 620 mm (D) × 830 mm (W) (see Fig. 18.11) and its weight is 195 kg.

Other products being developed include a 1 kW_el Kingston being developed by Genlec Energetix Ltd (Capenhurst, UK). This MCHP unit is based on organic Rankine cycle with a scroll engine (see Fig. 18.12) [2] and its overall and electric efficiencies are 90 and 10%, respectively.

Cogen Microsystems (Australia) has developed a Rankine cycle Cogen Micro unit based on a reciprocating steam engine (see Fig. 18.13) [3]. There are domestic (2.5 kW_el, 11/22 kW_th) and small commercial (10 kW_el and 44 kW_th) versions of Cogen CHP systems, both with an overall efficiency of 90%. Dimensions and weight of the domestic version are 870 mm (H) × 600 mm (W) × 400 mm (D) and 60 kg. The size of the commercial version is 960 mm (H) × 800 mm (W) × 600 mm (D) and its weight is 175 kg.

18.3.2 Internal combustion engine (ICE) MCHP

Currently, only the ECOWILL micro-CHP unit [4] is commercially available which is manufactured by Honda on the basis of the GE160V natural gas fuelled internal combustion engine. The engine is four stroke, water cooled single cylinder with overhead valves, with a cylinder displacement of 163 cm³. The system has 1 kW_el and 2.8 kW_th outputs with the option of an external auxiliary boiler. Dimensions of the system are 580 mm (W) × 380 mm (D) × 880 mm (H) and its weight is 83 kg. The overall efficiency of the system is 85.5% and power generation efficiency is 22.5%. It uses a catalytic converter along with an elaborate acoustic attenuation system to conform to the market requirements, although it might still not be suitable for indoor installation in its current configuration. In Japan a federation of three major organisations in the gas energy industry intends to sell a total of 235,000 units of ECOWILL by 2011. A variation of Ecowill, Freewatt, is sold in the USA. Freewatt has slightly greater power (1.2 kW_el) and heat outputs and is sold complete with a heating system: a boiler for hot water or a furnace for hot air [5].

There are several other ICE-based CHP units, such as Baxi SenerTec Dachs (Germany) [6] and XRGI 15 of EC Power (Denmark) [7] with an electrical power output greater than 4 kW_el, and these can be classified as mini-CHP units.

The Baxi Dachs Mini CHP system is based on a four stroke, water cooled single cylinder internal combustion engine, which was specially developed by Fichtel and Sachs AG (Germany). Electrical and thermal outputs are 5.5 kW_el and 12.5 kW_th, respectively, with the electrical and total efficiencies being 30% and 79-92%. Dimensions are 720 mm (W) × 1070 mm (D) × 1000 mm (H) and the weight is 530 kg. The service interval is 3500 hours.

XRGI 15 (see Fig. 18.14) is based on the four stroke, four cylinder water cooled Toyota gas engine. The swept volume of cylinders is 2237 cm³ and the engine is equipped with the oxidation catalyst. The electrical and thermal outputs are 6-15.2 kW_el and 17-30 kW_th, respectively. The total efficiency of the system is 92% with the electrical efficiency being 27-30%. Dimensions of the power system (only) are 1250 mm (H) × 750 mm (W) × 1110 mm (D) and the weight is 700 kg.

18.3.3 Stirling cycle MCHP

A number of Stirling engine-based units are being developed by Whisper Tech (New Zealand) [8], Disenco Energy (UK) [9], Microgen Engine Corporation (a consortium of the gas boiler companies Viessmann, Baxi, vaillant and Remeha with Sunpower) [10], Enatec Micro-Cogen (Netherlands) [11] and Sunmachine [12]. The nominal electrical and thermal outputs vary between 1-3 kW_el and 5-36 kW_th respectively.

Whispergen Mk 4/5 is shown in Fig. 18.15. This is built around a four cylinder double-acting Stirling engine with a wobble-yoke drive mechanism (see Fig. 18.16). The system has two burners: main and auxiliary. The main burner is to transfer the heat into the working fluid (nitrogen) inside the engine through heat exchangers integrated in the cylinder heads. The auxiliary burner is used to boost the heat output during large heat demand periods. Both burners are of a premix surface type. The generator is a four pole single phase induction generator. The electrical and thermal outputs are 1 kW_el and 5.5-7 kW_th (up to 13-14 kW_th), respectively. The dimensions of the MCHP unit are 491 mm (W) × 563 mm (D) × 838 mm (H) and its dry weight is 148 kg.

E-ON in the UK supplied a relatively small number of Whispergen Mk 4 and Mk 5 MCHP units to households in the south of England and currently work is going on with manufacturing partners towards a full market roll-out of mass-produced units [13]. Efficient Home Energy Sl (or EHE, based in the Basque region of Spain), which is a joint venture between Whisper Tech Limited (a subsidiary of the New Zealand company Meridian Energy Ltd.) and the Mondragón Corporation, has commenced a large-scale production of Whispergen MCHP units for the EU market (exclusive UK) [14].

A 3 kW_el MCHP unit from DISENCO is still in the development stage and little information on the technical specification is available on this product. Figure 18.17 shows a schematic of this system which has approximately the same dimensions as a Whispergen (the size of a domestic dish washer) and is built around a single cylinder beta-type Stirling engine which uses helium as a working fluid and runs at a speed of 3000 rpm. The engine used in this MCHP was derived from the engine developed by Sigma Elektroteknisk (Norway). In order to deliver 3 kW_el output, the engine should be pressurised to about 70-80 bar. A proposed thermal output is 15-18 kW_th (up to 30 kW_th in conjunction with a thermal storage) which makes the overall efficiency of the system 90%. Figure 18.18 schematically shows components of this MCHP unit.

The Microgen MCHP unit is built on the basis of the extensively modified free-piston Stirling engine, which was originally designed by SUNPOWER Inc (USA), and it has a 1 kW_el output [3] (see Fig. 18.19). Field trial results were encouraging, with the demonstration of net electrical efficiencies of over 15% in conjunction with an overall efficiency of 90%. Figure 18.20 demonstrates the wall-mounted Baxi Ecogen MCHP system [15] built around Microgen’s free-piston Stirling engine. The thermal output is 6-6.5 kW_th (up to 24 kW_th with an auxiliary burner), its dimensions are 920 mm (H) × 426 mm (W) × 425 mm (D) and the weight is 115 kg. The overall efficiency of the system is about 98% or greater (in condensing mode).

Enatec, in collaboration with Rinnai (Japan), Bosch Thermotechnik (Germany), Merloni TermoSanitari (Italy), plan to carry out Europe-wide field tests of their MCHP appliance. The system is built around a modification of a 1 kW_el free-piston Stirling engine originally developed by Infinia Corporation (USA) (see Fig. 18.21). Enatec is developing two versions of the MCHP system also called HRe boilers (see Fig. 18.22), one of which has a built-in heat storage tank. The thermal output is 4-35 kWth (when auxiliary burner is on), the overall efficiency of the system is > 98 in condensing mode.

Finally, a 3 kW_el Stirling engine MCHP unit is being built and sold by Sunmachine GmbH (Germany). It is built around a gamma (or alpha)-type Stirling engine with a wood pellet burner. The system is equipped with a 50 l-wood pellet storage tank. The electrical and thermal outputs are 1.7-3 kW_el and 6.5-10.5 kW_th. Electrical and overall efficiencies are 20% and > 85%, respectively. Dimensions of this wood pellet MCHP system are 1160 mm (D) × 760 mm (W) × 1590 mm (H) and its weight is 410 kg.

There is also the SOLO V161 Stirling CHP system (see Fig. 18.23) which was commercially available in Europe a few years ago [16]. This unit was produced by Stirling Systems GmbH and had 2-9.5 kW_el and 8-26 kW_th electrical and thermal power outputs, respectively. The SOLO V161 Stirling CHP was used for small commercial applications. The system was built around an alpha (V-type) Stirling engine which uses helium as a working fluid. The crankcase of the engine was unpressurised and separated from the internal gas circuit using special rod seals.

• electrical and thermal demand profiles which depend on climatic conditions; occupancy size and pattern; dwelling type and house fabric details; equipment used and attitude towards energy use

• the design of the MCHP system which can be specified by its capacity, heat/power ratio, efficiency and performance characteristics, thermal storage used (type, size, efficiency), electrical stores (type, size, efficiency); heat delivery system and control system and regime

• differential in fuel and grid electricity prices, emission factors of fuel and grid electricity; electricity export tariff

• installation and maintenance costs.

• Space heating-supplied by the central heating system with extreme seasonal variation, dependent on climatic conditions, dwelling design, heat delivery mechanism, occupancy patterns and perception of comfort.

•  Domestic hot water-mainly supplied by the central heating system, but sometimes by electric auxiliary heaters. Little variation with season, but dependent on household appliances, occupants’ attitude to energy use, occupancy size and occupancy pattern.

• Cooking-supplied by number of appliances which may be fuelled by different fuels. Slight variation with season but dependent on cooking appliances, occupants’ attitude to energy use, occupancy size and occupancy pattern.

• The house is sited in the south east of England, where the Whispergen units are being initially installed. The house’s front faces south with a set of double glazed windows installed on the main south and north- facing walls. There are four vinyl windows in each of two main walls with 1 m × 0.91 m and 2 m × 0.91 m dimensions and a 13 mm air gap. The interior wall has a finish made of 100 mm bricks and the house has a gypsum board ceiling and wooden floor. Thermal performance is in line with Part L of the 2000 building regulations [25].

• Internal design temperature is 21 °C.

• The house has a conventional hydronic central heating system consisting of a gas-fired central non-condensing boiler (h = 0.8) supplying a network of appropriately sized radiators and 150 litre hot water storage cylinder.

• Domestic hot water is supplied entirely from the hot water cylinder with appliances including hot fill washing machine, cold fill automatic dishwasher, standard shower, bath and washbasins.

• Cooking is with the use of gas-fired hobs, oven and grill supplemented by an electric microwave, kettle and toaster.

18.5.1 Space heating

In order to calculate the space heating requirements of the dwellings throughout the year, a model was constructed in ‘Hot 2XP’, a program developed by the CANMET Energy Technology Centre [26] to analyse energy flows within buildings. Taking into account all the appropriate thermal gains (solar, occupants, equipment, cooking), building details and historical weather data, the variations in space heating requirements have been predicted as illustrated in Fig. 18.24. As can be seen, there is a significant variation between the months with no supplementary heating required during the summer months June to September and annual heat demand is 13,172 kWh.

The program also calculates the required size of space heating system, based on the maximum rate of heat loss. The result for the semi-detached house model was 5 kW_th. This corresponds to the combined thermal output of the radiators when operating, fully open, at their specified supply and return temperatures, i.e. the magnitude of the maximum instantaneous thermal demand from the space heating system. The Whispergen Mk 3 unit has a thermal output of approximately 6 kW_th which meets the heat demand in the semi-detached house.

The conventional control system of a central heating system consists of a ‘time led’ operation, with defined periods coinciding with the occupancy, between which the system follows the heat demand within the property. The space heating demand is monitored by a simple bi-metallic thermostat positioned within the property. This leads to a profile with a heat-up period and cycling once the design temperature has been reached. The magnitude of the demand is varied depending on the positions of the individual radiator valves. A common energy saving addition to the system is thermostatic radiator valves (TRVs) which automatically modulate the output of individual radiators to match the requirement of the room. The majority use the principal of thermal expansion with an appropriate mechanism to steadily close the valve as the room approaches the desired temperature. They save energy by reducing the overheating of individual rooms and, if the boiler is capable of modulating its output, reducing the cycling of the boiler. In terms of the demand profile, TRVs lead to a shape illustrated in Fig. 18.25. The exact proportions and shape depend on a number of factors, such as the initial room temperature, the building’s thermal inertia, performance of the TRVs and current climatic conditions. In particular, the steady state will be subject to fluctuating thermal gains, e.g. solar or occupancy, and this will vary with the control system’s response to these. However, to simplify the modelling, the profile may be split into two stages as shown in Fig. 18.26: the initial heating period where the design temperature is reached followed by the steady state where the temperature is maintained.

As the occupancy pattern is based around full-time work, the dwelling will be empty for a significant period in the middle of the day during the week; therefore there will be two timed periods of operation. At the weekend, there is a less prescribed pattern with the likelihood that the property is occupied all day, resulting in a single long period of operation. The occupied periods correspond to the steady state part of the profile (area 2) in Fig. 18.26, when the design temperature is maintained. Therefore the system will come into operation earlier to actually raise the temperature to that required. Taking into account the marked difference between weekdays and the weekend, a representative profile for each type of day is required for each month. However, due to the symmetry of variation throughout the year, months with similar thermal demands can be grouped together so reducing the number of profiles required (see Table 18.2). Appropriate banding is shown in Fig. 18.27, which breaks the year into five different heating categories.

This grouping requires ten separate daily profiles for the dwelling. These are generated using the occupancy pattern in conjunction with the banded daily requirements and output ratings of the systems from the CANMET program. The results demonstrate there is a large variation in the steady state power required from the central heating boiler.

18.5.2 Domestic hot water

An average person in the UK uses 49 litres of hot water per day, with little seasonal variation [27]. The actual consumption is linked to the composition of the household, e.g. single or multiple occupiers, over or under 60 years of age, young or dependent children. It also depends on the equipment used and their specification, e.g. automatic washing machine, high flow rate power shower, etc.

For the prescribed scenario of a couple with two dependent children utilising an automatic dishwasher, the average daily consumption rises slightly to 50 litres per person, resulting in a household consumption of 200 litres per day. This requires an annual thermal input of 4245 kWh_th. As stated, this is supplied from the hot water storage cylinder within the gas-fired central heating system.

This thermal storage method uses a heat exchanger within the cylinder to transfer heat from the closed loop boiler circuit to the surrounding water in the cylinder when the central heating system is active. This is regulated by a thermostat on the tank which restricts the maximum temperature to 60 °C. Domestic hot water is drawn off the top of the cylinder, as required, which is then refilled from the cold header tank. This system effectively creates a buffer between demand and generation, providing hot water outside the operation of the central space heating system. The size of the buffer, i.e. storage cylinder, is governed by the overall system design and the demands placed upon it. The capacity specified in the scenario is 150 litres which is around average with a capacity of 8.73 kWh_th, assuming an increase of the temperature of water from 10 °C in mains to 60 °C in the cylinder. This part of the thermal demand forms the base load for the system as it demonstrates little seasonal variation, therefore, only two daily profiles are required; weekday and weekend. The same profiles will also be applied to both scenarios. To form the profiles using an event-based method, the uses and their requirements must be identified along with an appropriate schedule.

The majority of hot water is required for one of five main activities. These are summarised in Table 18.3 along with their proposed characteristics for the purpose of modelling. There are obviously further miscellaneous uses, e.g. car washing, which are not daily and make them difficult to integrate into the model. However, they are relatively small and can be eliminated without affecting the accuracy of the model.

The schedule and frequency of the above events can be calculated by considering the activities of a household throughout the day and the estimated daily usage of 200 litres. The two proposed profiles are shown in Fig. 18.28. As expected, the weekend profile is more distributed throughout the day whilst the weekday profile shows concentrations around the limited occupancy periods.

18.5.3 Cooking

Cooking requires high temperature inputs that are not supplied by a central heating system. However, the heat produced does contribute to the thermal gains of the property, slightly reducing the auxiliary space heating required. If the thermal loads due to cooking are met by electricity, the electrical demand profile will demonstrate large fluctuations due to the high power requirements of the thermal devices. As the main cooking appliances in the scenarios are gas-fired, the effects on the electricity profile will be less dramatic.

• Base load-a constant load of about 100 W_e due to the standby power consumption of most electrical goods, plus the cyclic load from refrigeration devices. This is present all day, throughout the year, with little variance.

• Biased load-a load that occurs on most days at similar or predictable times, e.g. lighting, television, cooking appliances. These arise through external influences or habitual behaviour.

• Elective load-a load operated primarily at the occupier’s discretion, e.g. washing machine, PC. These are harder to predict due to their more irregular nature.

• response to heat demand (start-up and run-down characteristics);

• maximum outputs (electrical and thermal);

• modulation capabilities;

• full and part load efficiencies;

• emission analysis.

• Ultrasonic heat meter (Landis Gyr⁺ 2WR5)-measures the flow rate through the radiators and the heat supplied.

• Data logging via RS232-PC connection to CHP unit itself to log main parameters including electrical output.

• Gas meter-monitors gas consumption to calculate overall efficiency.

• Gas analyser (RBR Ecom KD)-analyses exhaust composition using heated sample system with probe placed directly in the exhaust flue of the unit.

18.7.1 MCHP performance as a response to heat demand: start-up

Start-up characterirics of the MCHP unit were recorded from a number of separate cycles for a 30-minute period following a heat demand signal. In all these cycles the central heating system was starting from room temperature.

The results demonstrated significant variation in the time taken to reach a semi-steady state; between 10 and 20 minutes. All of the profiles have very similar shapes and gradients, hence the variation arises from the initial delay in the system before it appears to start. The source of this delay is believed to be internal diagnostics by the control system with dependence on external variables such as the air and current central heating water temperatures. From examining a single pair of electrical and thermal profiles, distinct periods in the start-up procedure can be identified, as shown in Fig. 18.30.

18.7.2 MCHP performance as a response to heat demand: run down

A similar comparison was made between the profiles recorded after the end of the heat demand. Governed by the control system, the unit operates on a low power level for approximately 30 minutes after the heat demand ceases. Compared with start-up, the profiles are equally consistent in terms of their shape and gradient, but much more so in regards to timing, with two shutdown procedures apparent depending on the power level prior to shutdown. Notable thermal energy is supplied after the shutdown of the engine as the heat stored within the cylinder walls is transferred to the exhaust heat exchanger by the continuing operation of the burner fan. An examination of the run-down characteristic is given in Fig. 18.31.

18.7.3 MCHP performance as a response to heat demand: maximum outputs

The maximum outputs were measured by operating the system at 75 °C with the radiators fully open for a three hour heat demand period. This produced the profiles shown in Fig. 18.32. The thermal power reached 80% of its maximum after 20 minutes, plateauing at approximately 6.25 kW after 2 hours while the electrical power reached its steady peak of 850 W after 20 minutes. From examination of the gas consumption, this consisted of a single burner firing rate with the variation due to the system ‘warming up’.

18.7.4 MCHP performance as a response to heat demand: modulation capabilities

As the thermal demand within the household will vary and often be less than the unit’s maximum output, it is important to analyse modulation capabilities of the system. To investigate this, two of the radiators were three-quarters closed, the set point temperature reduced to 65 °C and a two hour heat demand period programmed. This produced the profiles shown in Fig. 18.33. From these results, it becomes apparent that the unit can only operate continuously on two separate power levels. In this run, the unit operated at full power to bring the flow temperature close to that of the set point, after which it switched to a lower level. However, this still provided too much thermal energy causing the flow temperature to continue rising. This resulted in the engine switching off entirely for a 12-minute period while the temperature dropped. The pattern was then repeated when the engine restarted.

The unit is capable of little modulation, with only steady state thermal outputs of 5.3 kW_th and 6.25 kW_th. This will produce a high cycling rate during long periods of low thermal demand which will be detrimental to the performance of the unit.

18.7.5 Efficiency per cycle and power level efficiencies

The economic performance of the unit will be governed by both its overall efficiency and heat-to-power ratio (HPR). This is due to the significant difference in gas and electricity prices. The efficiency and HPR can be examined on two levels:

For comparison, the profiles from Figs 18.32 and 18.33 were analysed along with their gas consumption data. The results are presented in Table 18.5. The figures show a significant difference in both the efficiencies and ratios for the compared cycles. As anticipated, the modulated cycle with its engine shutdown period is nearly 10% less efficient. Also, the proportions of heat generated are greater, further decreasing the comparative economic value.

The instantaneous efficiency of the unit was analysed for the two steady state power levels, namely full and intermediate power levels. This was achieved using two 3 hour runs at 65 °C and 75 °C and their gas consumption. The data used to calculate the efficiency were taken once the outputs had stabilised. The results are shown in Table 18.6. Both power levels have high overall efficiencies of over 90% but the major difference lies in the heat- to-power ratio. This is almost doubled when the unit switches from full to intermediate power. Hence, the possible economic returns will be severely diminished if the unit operates at the intermediate power level for extended periods due to the reduced proportion of electricity produced.

18.7.6 Emissions analysis

As one of the major drivers behind micro-CHP development is the reduction of harmful gas emissions, the composition of the Whispergen’s exhaust is very important. This was investigated using the gas analyser included in the test rig. The combustion process inputs are natural gas and air. Therefore, the components of flue gas are primarily made up of compounds of oxygen, nitrogen, hydrogen and carbon. The efficiency of the combustion process can also be determined from the combustion products. This was achieved using the oxygen and/or CO₂ measurement in conjunction with pre- and postcombustion gas temperatures. The calculation was performed automatically by the gas analyser. The exhaust composition and combustion efficiency was logged at two-minute intervals for a number of experimental runs. An example from a three hour run is shown in Fig. 18.34. The gas analyser relies on an electrochemical reaction for its readings and must be purged at regular intervals during continuous measurement, hence the short gaps in readings.

The actual amount of carbon dioxide produced depends on the type and amount of fuel burnt, hence this was a function of gas consumption rather than the burner design. For natural gas, this was 0.19 kg/kWh. Carbon monoxide is the product of incomplete fuel combustion from the lack of oxygen. As the Stirling engine design is based on continuous external combustion, which is easily optimised, the combustion process is highly efficient as indicated. This results in little formation of carbon monoxide once the process has stabilised.

The principal source of NO_x emissions in natural gas appliances is the oxidation of excess atmospheric nitrogen at temperatures above 1100 °C. Modern applications require the minimisation of NO_x through advanced burner design. Emissions can be greatly reduced by rapid or complete mixing of the gas/air fuel supplies and encouraging the internal recirculation of combustion products. This minimises the peak temperatures in the combustion zone. The Whispergen Mk 3 unit has peak NO_x emissions of over 120 ppm. During steady state operation this level is about 80 ppm. Typical NO_x emission level in conventional burners is between 110 and 150 ppm. In burners with the fully pre-mixed combustion process, the NO_x emission level is reduced to 45-75 ppm whilst advanced burners, such as catalytic stabilised and catalytic, have NO_x emissons less than 5-10 ppm [30].

• Thermal inertia-As well as the thermal mass of the house and hot water cylinder, the water and associated pipe work within the heating circuits have their own mass. This affects the lag present within the system. The heat capacity of the test rig was determined by examining the rate at which the radiators heated during the experimental runs in conjunction with the thermal input and dissipation calculated. This capacity was then scaled to each scenario according to the maximum space heating output of each system. Additional thermal mass was also added to model the 5 kW heating circuit through the hot water cylinder which is not present in the test rig.

• Individual space and tank thermostats-Most heating systems contain a distribution valve which directs the hot water from the boiler to where it is needed. If no further space heating is required, all the water from the boiler is directed to the hot water cylinder and vice versa. This was included in the model along with the effects when the valve position changes, mixing standing water from one circuit with the active circuit.

• Variable heat loss-As the inside of the house heats up, the heat loss through the fabric will increase in proportion. The heat losses from the house in various seasons, at design temperature, were obtained from the CANMET program results. These were then used to model variable heat loss for each season based on the current internal and appropriate ambient temperatures.

• Standby losses from hot water cylinder-While the hot water is stored in the cylinder, an amount of heat is lost through the walls depending on the temperature of the water. This can be reduced through insulation, although not totally removed. An appropriate loss was included in the model in line with current British Standards for hot water cylinders. The effect of thermal stratification within the cylinder was also taken into account for modelling the performance of the heat exchanger.

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