The system under investigation – has been installed at an office building in the South-East of England. The site has energy requirements for heating and cooling. To satisfy the energy requirements, a 95 kW_e, 165 kW_T Perkins (de-rated 110 kW_e) ‘hot-engine’ unit was used together with a 200 kW_T Carrier absorption-chiller with a of approximately 0.70. The absorption chiller was installed in series with two 210 kW_T vapour-compression ( ) units, and operated as the lead chiller, with the units switched on and off as required. The system was fitted with an air-blast cooler, which requires greater amounts of electricity and is more expensive than a standard ‘wet-cooling-tower’. The ‘air-blast’ cooling system is used as an alternative to the ‘wet-system’, in order to reduce maintenance costs and prevent the risk of Legionella bacteria forming in the evaporating water. The effectiveness of this integrated small-scale and absorption chiller system has been assessed in terms of the carbon-dioxide emissions produced for each kWh of coolth (kWh_T) delivered. Several assumptions concerning the system and the operation of its components have been made for simplicity and to ensure that like-for-like comparisons are made where possible.

The following four systems have been studied in detail:-

I. 95 kW_e, 165 kW_T unit + 196 kW_T absorption chiller + 129 kW boiler.

II. 294 kW boiler + 200 kW_T absorption chiller.

III. 164 kW_e + 294 kW_T absorption chiller.

IV. Electrically driven vapour compression unit, using refrigerant 134a.

System I represents the integrated and absorption units as they are installed at the site. System III has a larger, 164 kW_e, 294 kW_T unit installed, so that it can supply all the heat required by the absorption chiller. The heat required for the absorption chiller in System II is provided only by a gas-fired boiler, with the electricity for the parasitic loads taken from the national grid. System IV uses a similarly-sized electrically-driven vapour-compression unit, to supply the desired cooling. The power required for the parasitic load in the non- cases is provided by the grid.

The electricity consumptions of each of the parasitic loads (i.e. the powers to drive the fans and pumps) in the system have not been measured accurately. Therefore, it is not possible to determine the exact power-consumption of the system. However, Table 2 shows the electrical ratings of the pumps and fans. Table 3 gives an indication of the likely power-consumptions for the specified load and equipment conditions, when considering the mass-flow rates of the fluids and the pressure-drop across different sections of the system. The predicted figure of 11.4 kW falls well short of the combined power-ratings of each of the individual pumps in the system (at maximum load). However, it is unlikely that the system will operate at full-load conditions for more than a few hours each week.

To analyse the full range of operating conditions, a parametric analysis has been undertaken, with the power consumption of the parasitic loads varying as seen in Table 4. The cooling and chilled-water pumps in the air-conditioning system are set up for single-speed and constant-flow operation. The cooling-water pump power (CWPP) is 5.5 kW and the chilled-water pump power (ChWPP) is 2.2 kW. The two internal refrigerant and solution pumps incorporated within the absorption chiller also work at constant power: the solution and refrigerant-pump power (SRPP) is 3.8 kW. Only the fan power (FP) and Generator-pump power (GPP) fluctuate according to demands i.e.
6.4 kWFP14.8 kW,

(3)

4.5 kWGPP11.0 kW.

Table 4 presents the range of power consumptions from P_min to P_max for the integrated plus absorption chiller and vapour-compressor chiller. It is assumed that increasing the electrical-power consumption of one of the components in the system will mean that additional power is required for the other pumps. This is a reasonable assumption as the individual processes in the integrated system are linked.

The parasitic power consumption for the absorption systems and resultant hourly CO₂ emissions will be assumed to lie in the ranges respectively:

(6)22.4 kWP37.3 kW,

(7)22.2 kg/hCO₂36.9 kg/h.

For systems, it can be assumed that the parasitic power consumption and resultant CO₂ emissions will lie in the ranges:

(8)8.2 kWP12.4 kW,

(9)8.1 kg/hCO₂12.3 kg/h.

The for the absorption chiller is predicted without the inclusion of the energy expended to satisfy the parasitic loads.

In the present investigation, the minimum power P_min, which is considered as a realistic estimate by the site’s operators for the various pumps requirements, will be taken as the parasitic load for both the absorption-chiller and the vapour-compression systems. The CO₂ emissions for systems (1)?(4) are predicted under several different operating conditions. Initially, the level of carbon-dioxide emissions per kWh_T is examined for the four separate systems with parasitic loads for the absorption-chiller and vapour-compression systems assumed to be 22.4 and 8.2 kW respectively. An analysis of the effect of increasing the parasitic power consumption is then undertaken. Finally, the rate of CO₂ emissions for varying source and cooling-water temperatures will be determined for minimum and maximum parasitic-power loads.

The level of CO₂ emissions per kW_T for each of the four considered systems will be determined from the sum of one or more of the following:-

(i) The CO₂ emissions for the fuel burnt associated with that proportion of electricity and heat, which are provided simultaneously by the unit to the absorption chiller, to satisfy both the electricity demand required by the parasitic load and some of the heat demand. The electrical and thermal efficiencies of the units considered are 29.6% and 51.1% respectively.

(ii) The CO₂ emissions associated with the production of the heat, which is used to drive the absorption chiller, from the unit. The remaining unused electricity from the unit is set against the electricity produced in a steam turbine with a generating efficiency of 35%. The difference between the two systems, in terms of electricity generated, is allocated as the cost (representing the CO₂ emissions associated with fuel burn) of the remaining heat supplied by the unit.

(iii) The CO₂ emissions associated with the supply of the remaining heat, required to drive the absorption chiller, from a gas-fired boiler (of efficiency 80%).

(iv) The CO₂ emissions associated with the central generation of electricity used to drive the parasitic loads and/or the vapour-compression chillers.

Two examples of the system with a unit and an absorption chiller are considered. In the first case, which represents the site as it is currently set up, only part of the heat demand from the chiller will be supplied by the unit, whereas, the unit in System III supplies all of the heat required.

System I: plus absorption chiller plus a 129 kW boiler. This system operates with a 95 kW_e, 165 kW_T de-rated gas-fired engine and requires an additional heat-input of 129 kW from the boiler in order to maintain the desired cooling-effect (196 kW_T) from the absorption chiller. External auxiliary equipment (having parasitic loads), such as the chilled-and-cooling water pumps and fans will require the expenditures of further electrical power as follows: refrigerant pump (2.2 kW), cooling-water pump (5.5 kW) and generator pumps (total 11 kW). In addition to pumping power, energy will be required for the cooling fans, which have a maximum rating of 14.8 kW. The solution and refrigerant pumps, operating within the absorption-chiller unit, will require a further 3.8 kW, which will result in a total maximum parasitic load of approximately 37.3 kW. However, the fans are rarely operated at maximum power and their electrical consumption is on average assumed to be 6.4 kW. The electrical consumption of the generator-pump is directly proportional to the amount of cooling output demanded by the site. The average parasitic load for the system is assumed to be 22.4 kW_e as detailed in Table 4.