Micro-turbine CHP Units: Simulations of Energy Efficiency and Cost in Ornamentals Production

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1 Micro-turbine CHP Units: Simulations of Energy Efficiency and Cost in Ornamentals Production P. J.C. Hamer 1 F. A. Langton Silsoe Research Institute Warwick HRI Wrest Park University of Warwick Silsoe, Bedford Wellesbourne, Warwick MK45 4HS, CV35 9EF United Kingdom United Kingdom Keywords: Temperature integration, energy balance models, bedding plants, CO 2 emissions, supplementary lighting Abstract CHP is the simultaneous production of heat and electricity. For the ornamenttals grower, the micro-turbine CHP has the potential to be used as an energy efficient method of generating electricity for supplementary lighting, and for providing heat and CO 2 for crop production. In addition, CHP can make a significant contribution to Government energy policy by reducing CO 2 emissions. Simulations were carried out using an energy balance model for a typical ornamentals lighting installation based on a CHP system comprising a micro-turbine CHP unit, a heat store and a back-up boiler. Energy profiles were compared with those given by a conventional system comprising a boiler to provide the heat, and taking electricity for the lamps from the National Grid. Separate simulations were conducted for four different lighting regimes, each with and without temperature integration (an energy-saving protocol). The study used current UK fuel costs. The simulations indicated that CHP can give running cost savings of 3% to 42%, with the higher savings achieved at long operating times. Furthermore micro-turbine CHP reduced CO 2 emissions by between 25% and 35%. Temperature integration can save energy, particularly for the shorter lighting period. However, for the grower the micro-turbine is not a cost-effective means of providing energy unless the existing mains supply is not large enough to meet the power demand. This is because the repayment of the capital investment is large and similar to the running costs. Furthermore the potential benefit of increased availability of CO 2 is not realised because the CHP is frequently not running at times when CO 2 would have benefitted production. INTRODUCTION Combined heat and power (CHP) is the simultaneous production of heat and electricity. Conventional electricity generation has the unfortunate, environmental consequence that the heat and CO 2 are rejected to the atmosphere. In contrast, when electricity is generated on a grower s holding using a CHP, the heat can be captured in a heat recovery system to produce hot water for heating, and CO 2 from the exhaust gas can be made available for enrichment to increase growth. In horticulture, large scale CHP schemes have been installed by power generators who operate them to provide electricity to the national grid and sell the heat and CO 2 to the grower. These installations are based around a natural gas fuelled reciprocating engine. The micro-turbine CHP is an alternative technology and has advantages over the reciprocating engine, which include lower electricity generation costs in the sub 5 kw range, reduced maintenance cost and cleaner exhaust gases so that expensive flue gas treatments are not needed (HDC, 22). However, the high cost of connecting a micro-turbine CHP to the national grid for export of electricity is usually prohibitive, and therefore all of the electricity generated has to be used on the nursery. For this reason, small stand-alone CHP units appear best suited for 1 Current address: 15E Bow Brickhill Road, Woburn Sands, Milton Keynes, Bucks., MK17 8QD, United Kingdom. paulhamer@onetel.com Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS

2 energy saving by ornamentals growers who require electricity for supplementary lighting, and can potentially use the CO 2 exhaust gas for enrichment. All of the energy generated by the CHP is used on site, and the CHP only operates when the supplementary lights are on, since there is no cost-effective way of storing electricity. To assist in meeting its Kyoto Protocol target of an overall 12.5% reduction in UK greenhouse gas emissions, the UK Government has introduced a variety of incentives. These include a Climate Change Levy (CCL) on greenhouse heating fuels, but this has not been imposed on fuel used for CHP generation. Advantageous tax breaks have also been offered on the cost of purchase of CHP. In this paper, an energy balance modelling tool is used to assess by simulation the economic viability of operating a micro-turbine CHP unit for ornamental crops, and the potential reductions in emissions of CO 2. MATERIALS AND METHODS In ornamentals production, a typical lighting installation consists of 4 W SON/T lamps, each lighting an area of 11.2 m 2. Only 18 W of the 435 W of electrical energy required to light a 4 W SON/T lamp is actually converted to light (PAR, 4 7 nm), so the irradiance is 9.6 W/m 2 (Bot et al., 1995). The larger part, about 33 W, is dissipated as heat into the glasshouse reducing the amount of energy required (q, W/m 2 ) by the heating system. When heating is required, the rate of energy supplied, q, by the heating system can be written as a modification to an energy balance model (Bailey, 1988) to take account of H l (W/m 2 ), the heat from the lamps: q = UA( Ti To) ( H + Hl) (1) Methods for estimating H (W/m 2 ), the sensible heat as a proportion of the incoming irradiance, and U (W/(m 2 K)), the overall heat transfer coefficient, are presented elsewhere (Defra, 23). A is the ratio of greenhouse cover to ground area; T i (K) and T o (K) are temperatures inside and outside the greenhouse. Currently available micro-turbine CHP units produce 1.75 units of heat (Q h ) for every one unit of electrical output (Q e ), i.e. Q h /Q e =1.75 (HDC, 22). Rarely are the electrical and heat requirements on the nursery in balance with the electrical and heat outputs of a CHP unit. Where heat from the CHP is in excess of the demand, the surplus heat can be stored as hot water for use later, but when there is insufficient heat (either from the CHP or the heat store) the extra heat has to be supplied from a separate (conventional) boiler. Therefore for this study a system is considered to comprise a micro-turbine CHP unit, a heat store and a conventional boiler. The CHP unit only operates when there is an electrical demand for supplementary lighting. At any time, the heat produced is either in excess of the requirement and goes into the heat store, or there is a shortfall in which case heat is taken from the store. However, if the store is empty, the boiler operates to produce sufficient heat to meet the demand. The size of the store was limited to a maximum of 4 MJ/m 2 (Bailey et al., 2) and any surplus heat was assumed to be dumped. Energy use simulations were run with hourly data of external air temperature wind speed (to calculate a value of U) and solar radiation at Cardington, Bedford, UK (52.2N;.42W) for the years 1972 to Running the model for each of the years separately enabled the means and standard deviations of energy use and energy savings to be computed. A greenhouse block of 5m 2 was assumed, with 5 spans each 8m wide and 125m long, and height to the eaves of 4.3m. The ratio of greenhouse cover to ground area was A = Profiles of energy use were simulated for four lighting regimes, each with and without temperature integration (TI). This is an energy-saving protocol with a high vent temperature set-point, that gives higher day temperatures than are usually tolerated, allowing lower than usual set-point temperatures at other times (for a background to this 634

3 see Defra, 23). For the no TI protocol, the day and night heating set-points were 18 C and the venting set-point was 23 C. For the TI protocol, the 24-hour average temperature was set at 18 o C and the venting set-point at 26 C. Day and night set-point temperatures were allowed to fall as low as 12 o C when an excess temperature sum had accumulated. Integration was over a rolling, 5-day period. The following lighting systems were studied: Case 1a chrysanthemum production regime with supplementary lighting for 6 months/year. It was assumed that a thermal screen was in place between sunset and sunrise and used as a blackout screen to provide an 11.5 hour day between 6:3 and 18:, the times taken for the change-over from day to night set-points. Lights were on between these times for the six month period from October to March unless the internal irradiance level was greater than 15 W/m 2. Case 1b chrysanthemum production regime with year-round supplementary lighting. The lights were on throughout the year with all of the settings, including the screen and temperature control set-points, the same as in Case 1a. Case 2a non-photoperiodic or long-day regime with lights on for 16 hours per day. A thermal screen was assumed to be in place between sunset and sunrise, and lights were on throughout the year for 16 h/day between 4: and 2: whenever the internal irradiance level was less than 15 W/m 2. The temperature control conditions were the same as for Case 1b and the demarcation between day and night was determined by the lighting period (i.e. 4: and 2:). Case 2b non-photoperiodic or long-day regime with lights on for 2 hours per day. This was the same as Case 2a except that the lights were on for 2 h/day between 2: and 22: whenever the internal irradiance level was less than 15 W/m 2. Whenever the lights were on the CHP unit was operational. The potential reductions in emissions of CO 2 by the heating system were calculated by comparison with a conventional heating system comprising a boiler supplying the heat to the greenhouse and the national grid supplying the electricity. It was assumed for both systems that the boiler was fired by natural gas. A carbon intensity factor of.43 kg CO 2 per kwh of electrical consumption was assumed, a value used for calculating carbon reductions ( The CO 2 produced by the boiler assumed a boiler efficiency of 85% and a calorific value of natural gas of 36.5 MJ/m 3. The costs of running a CHP unit were estimated using procedures outlined in (HDC, 22) and assuming the following current energy and equipment costs: Repayment on capital 146 per kw.annum, based on an installed cost of 7 per kw e with a repayment period of 6 years and interest on capital of 6.75%. Maintenance.7 p/kwh e Natural gas.85 p/kwh Climate change levy.15 p/kwh on gas used by the boiler and none on gas used by the CHP unit Grid electricity 5. p/kwh RESULTS Simulation of CHP operation for chrysanthemum production and year-round lighting (Case 1b) indicated that the supplementary lights were on for most of January and December (around 34 h/month), as the irradiance in the greenhouse was generally below the set-point value of 15 W/m 2 (Fig. 1). The operating time reduced to 85 h/month in mid-summer. This was not affected by whether TI was used or not. The heat produced by the CHP unit was insufficient to meet all of the demand, so the boiler was needed throughout the year when TI was not used, and was used for all except the summer months when TI was used (Fig. 2a). With TI, the demand for the boiler was very small in the summer, and there was excess heat to be dumped (Fig. 2b). 635

4 For other non-photoperiodic or long-day crops, the boiler needed to provide about 5 MJ/m 2 per month in the winter when the lights were on for 16 h/d (Fig. 3a) but this reduced to 2 MJ/m 2 per month for 2 h/d lighting (Fig. 4a). The excess heat peaked in the late summer/autumn, and increased with the duration of lighting, and further increased for TI control. The CHP operating time was governed entirely by the duration of lighting and was not affected by whether TI was used or not. In general, the greater the annual CHP operating time, the smaller the requirement for additional boiler heat (Table 1). In all cases studied, the use of TI reduced boiler demand (by between 11% for case 2b and 25% for cases 1a and 1b). The emissions of CO 2 were estimated by calculating the total CO 2 produced by burning fuel (Table 2). Note that there was no deduction of CO 2 used for growth, since such CO 2 is ultimately released into the atmosphere when the plants die and decay. The reduction in emissions of CO 2 utilising a micro-turbine CHP unit was typically in the range 25-35%. The running costs associated with a CHP system were found to be considerably less than for a conventional boiler system (Table 3). Thus, savings on running costs were: 3% (case 1a), 36% (case 1b), 42% (case 2a) and 4% (case 2b). In practice, the electricity costs associated with cases 2a and 2b would be somewhat lower than presented, since it is probable that off-peak rates would apply for part of the daily lighting period. However, when the costs of capital repayment of the CHP unit were included in the calculations, the conventional system showed up as much more cost-effective. Benefits of the conventional system over the CHP system in total costs ranged from 2% (case 2b) to 32% (case 1a). DISCUSSION AND CONCLUSIONS The reduction in emissions of CO 2 (25-35%) by the utilisation of a micro-turbine CHP unit to grow ornamental plants indicates that CHP units clearly are environmentally friendly and can, therefore, make a significant contribution to Government energy policy. Currently, however, purchase costs make micro-turbine CHP units economically non-viable for the average grower who has access to a mains electricity supply that is large enough to meet requirements for lighting. This would, however, change if electricity prices increased. For the cases studied, increases in electricity prices of between 14% (for case 1a) and 2% (for Case 2b) would change the viability in favour of CHP (assuming no increase in gas prices). For cases 1b and 2a, increases in electricity prices of 55% and 13% respectively would be required. Operating a CHP unit with TI can save energy, as shown for example in Fig. 2a for case 1a. However, the benefits of reduced energy consumption by TI are considerably diminished in absolute terms when operating times increase since the lamps continue to supply heat even when the minimum set-point temperature has been exceeded. Thus, much of the energy that is potentially saved by TI has to be destroyed unless it can be used elsewhere on the nursery. The potential benefit of increased availability of CO 2 is also not fully realised. When the CHP unit is operating (i.e. when the lights are on ) the CO 2 produced is considerably in excess of the requirement for enrichment. The CO 2 produced by a CHP unit is.66 kg/h for each kw of electrical output (HDC, 22). For the cases presented, the electrical requirement is 435 W for lighting an area of 11.2 m 2, i.e W/m 2. The CO 2 produced is 256 kg/(ha.h) and is considerable in excess of the rule of thumb requirement of 5 kg/(ha.h) for enrichment (HDC, 22). There is no means of storing the CO 2 although the excess can be utilised in other parts of the nursery. The lights are off at times when the light inside the greenhouse exceeds 15 W/m 2 (equivalent to about 25 W/m 2 outside). Thus, there is no CO 2 available for enrichment at times when the plants would benefit most from CO 2 enrichment. If there is a requirement for heat then this is met first from the heat store and, again, this means no CO 2 available until the heat store is empty. Therefore, including a micro-turbine and a heat store into a heating system reduces 636

5 the opportunity to enrich with CO 2 when benefits are likely to be greatest i.e. when light levels and temperatures are high. ACKNOWLEDGEMENTS This project HH133 was supported by the UK Department for Environment Food and Rural Affairs (Defra). Literature Cited Bailey, B.J Control strategies to enhance the performance of greenhouse thermal screens. Journal of Agricultural Engineering Research 4: Bailey, B.J., Hamer, P.J.C., Virk, G.S. and Ford, M.G. 2. Novel methods for heating and cooling greenhouses. Silsoe Research Institute, Contract Report CR/1119//283, 33 pp. Bot, G.P.A., Challa, H. and Van de Braak, N.J Greenhouse climate control an integrated approach. Wageningen: Wageningen Pers, 276 pp. Defra 23. Energy efficient production of high quality ornamental species. SPC_1311_FRP.doc. HDC 22. Micro-turbine CHP Units: Their application in protected horticulture. Horticultural Development Council, A Grower Guide, 23 pp Tables Table 1. Utilisation of CHP and boiler, with and without temperature integration (TI). The value is the mean and ± the standard deviation over different years. Case Annual CHP Annual boiler heat requirement (MJ/m 2 ) operating time (h) With TI 1a 172 ± ± ± 56 1b 2415 ± 7 63 ± ± 58 2a 3967 ± ± ± 68 2b 5427 ± ± ± 44 Table 2. Comparison of the average annual CO 2 emissions (kg/m 2 ) from a conventional boiler heating system and a system including a micro-turbine CHP. The CHP unit was used whenever the lights were on, and temperature integration was not used. Case Boiler + grid Micro-turbine CO 2 Reduction electricity CHP + boiler (%) 1a b a b

6 Table 3. Comparison of average annual costs ( /m 2 ) from (a), conventional boiler heating system where electricity for the supplementary lighting is provided from the national grid and (b), the micro-turbine CHP with a boiler used as a back-up. There is no temperature integration control. (a) Boiler + grid electricity Case 1a Case 1b Case 2a Case 2b Boiler gas Electricity Total running cost (b) CHP + boiler Case 1a Case 1b Case 2a Case 2b CHP gas CHP maintenance Boiler gas Total running cost Capital repayment Total cost Figurese 4 35 CHP operating time, h/mth Fig. 1 Case 1b. Mean monthly CHP operating times. Bars indicate the standard deviation over different years. Boiler heat, MJ/(m 2.mth) (a) Excess heat, MJ/(m 2.mth) (b) Fig. 2. Case 1b. Mean monthly (a) heat required from the boiler and (b) the excess heat when the lights are on for 11.5 h per day. Bars indicate the standard deviation over different years. 638

7 (a) (b) Boiler heat, MJ/(m 2.mth) Excess heat, MJ/(m 2.mth) Fig. 3. Case 2a. Mean monthly (a) heat required from the boiler and (b) the excess heat when the lights are on for 16 h per day. Bars indicate the standard deviation over different years. (a) (b) Boiler heat, MJ/(m 2.mth) Excess heat, MJ/(m 2.mth) Fig. 4. Case 2b. Mean monthly (a) heat required from the boiler and (b) the excess heat when the lights are on for 2 h per day. Bars indicate the standard deviation over different years. 639

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