Neil Hewitt

Techno-economic assessment of biofuel development by anaerobic digestion of European marine cold-water seaweeds

The techno-economic characteristics of macro-algae utilisation from European temperate zones was evaluated in a selected Anaerobic Digester (AD) using the chemical process modelling software ECLIPSE. The assessment covered the mass and energy balance of the entire process followed by the economic feasibility study, which included the total cost estimation, net present value calculation, and sensitivity analysis. The selected plant size corresponded to a community based AD of 1.6 MWth with a macro-algae feed rate of 8.64 tonnes per day (dry basis). The produced biogas was utilised in a combined heat and power plant generating 237 kWenet electricity and 367 kWth heat. The breakeven electricity-selling price in this study was estimated at around €120/MWh. On the ground of different national and regional policies, this study did not account for any government incentives. However, different support mechanisms such as Feed-in-Tariffs or Renewable Obligation Certificates can significantly improve the project viability.

Advanced cycles and replacement working fluids in heat pumps

Heat pumps will continue to make a strong positive contribution to the reduction of carbon dioxide emissions associated with energy production. They are energy efficient under certain conditions and also cost effective (especially when displacing electric heating). It is this problem of cost effectiveness that affects market penetration and limits their use. One method of improving the payback period is by improving the efficiency so as to increase the energy savings, and thus the cost savings. This also has, of course, a positive effect on the environment. This paper examines a number of alternative fluids and systems in an attempt to improve performance of heat pumps for both space heating and industrial processes.

Composition shift in liquid-recirculation refrigerating systems: an experimental investigation for the pure fluid R134a and the mixture R32/134a

Abstract The ability of zeotropic mixtures with a remarkable temperature glide to operate in liquid-recirculation systems is investigated and the results of an experimental comparison between the performances of the pure fluid R134a and the zeotropic mixture R32/134a (25/75% by mass) are presented. R134a performs slightly better in the liquid-recirculation mode than in the traditional dry-expansion mode; on the other hand, liquid-recirculation configuration has a detrimental effect on the zeotropic mixtures performance. The reason for this detrimental effect is the mixture component separation which occurs at the liquid/vapor separator. The effect of this separation is investigated using gas chromatograph analysis.

The replacement of CFCS in refrigeration equipment by environmentally benign alternatives

Abstract The replacement of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) is of prime importance since these refrigerants make up the vast majority of the working fluids used by the refrigeration industry and suitable alternatives must be found. A number of suitable pure fluids were considered to establish their properties and, in the case of flammable fluids, a risk analysis was carried out in order to assess their safety. The properties of their mixtures were calculated and suitable equations and p-h charts presented. Compressor lubricants have had to be changed since the majority of the CFC and HCFC replacements are hydrofluorocarbons (HFCs). Typically, HFCs are immiscible in the traditional mineral oils and thus a new generation of synthetic oils (usually polyol ester based) have been produced. The properties of these oils have been investigated and various models, including a modified Flory-Huggins equation, have been used to describe their solubility in refrigerants. The effects of refrigerant/oil solubility of the pure refrigerants and their mixtures on system performance have also been analysed and, in the case of refrigerant mixtures, there is a possibility of a composition change due to the differential solubility which influences performance. Finally, compact plate heat exchangers were found to be excellent heat exchangers for the refrigeration industry, provided that it is remembered that there are control and capacity variation limitations associated with their use. The falling film evaporator is studied in detail and there is a possibility that it will fulfill its potential. Compressors tested with alternative refrigerants have shown (in the case of R407c, a replacement for R22) a deterioration in performance with decreasing evaporator temperature when compared to that of R22, while some of the flammable refrigerants may require larger compressors than the CFC and HCFC counterparts, due to their smaller mass flow rates.

Waste biomass from production process co-firing with coal in a steam boiler to reduce fossil fuel consumption: A case study

Abstract Waste biomass is always generated during the production process in industries. The ordinary way to get rid of the waste biomass is to send them to landfill or burn it in the open field. The waste may potentially be used for co-firing with coal to save fossil fuel consumption and also reduce net carbon emissions. In this case study, the bio-waste from a Nicotiana Tabacum (NT) pre-treatment plant is used as the biomass to co-fire with coal. The samples of NT wastes were analysed. It was found that the wastes were of the relatively high energy content which were suitable for co-firing with coal. To investigate the potential and benefits for adding NT wastes to a Fluidised Bed Combustion (FBC) boiler in the plant, detailed modelling and simulation are carried out using the European Coal Liquefaction Process Simulation and Evaluation (ECLIPSE) process simulation package. The feedstock blending ratios of NT waste to coal studied in this work are varied from 0% to 30%. The results show that the addition of NT wastes may decrease the emissions of CO 2 and SO x without reducing the boiler performance.

An economic and technical case for a compressor/expander unit for heat pumps

Despite their obvious environmental benefit, heat pumps in general have difficulties in penetrating the heating market because of the high initial capital cost. However, given the fact that in the U.K. space heating alone accounts for nearly 50 per cent of all the primary energy used, the universal adoption of heat pump subsidies similar to those offered by certain German utilities for example would be an effective contributor to the reduction of greenhouse gas emission. However, it would be improper for subsidies alone to be the only mechanism by which heat pumps could gain the scale of market acceptance necessary in order to make the required greenhouse gas emission cuts. Therefore, a new generation of heat pumps must have a superior performance than existing units operating with HCFC R22 or R407c. The use of R410a can aid heat pump market penetration by having heat pumps of a superior performance and a smaller size. However, to make full use of R410a, the difficulties of the relatively low critical point must be overcome and the use of a novel compressor/expander unit is illustrated. Copyright

From pure fluids to zeotropic and azeotropic mixtures : the effects of refrigerant-oil solubility on system performance

This paper highlights the effects of circulating compressor lubricants on the performance of vapour compression refrigeration systems. It will give a baseline case of R12 and a mineral oil and show to what degree the alternative refrigerants and their mixtures conform to this.

Refurbishing the UK's 'hard to treat' dwelling stock: Understanding challenges and constraints - the work of Project CALEBRE

Project CALEBRE (Consumer Appealing Low Energy technologies for Building REtrofitting) is a four year £2 million E.ON/RCUK funded project that is investigating technologies and developing solutions for the UK’s solid-wall houses to offer energy demand reduction, energy efficient heat generation and energy management combined with user appeal. Understanding how technical solutions can be aligned with householder lifestyles is central to the CALEBRE project. The technologies include: vacuum glazing to achieve exceptionally low U-values whilst being capable of retrofit in existing window frames; advanced gas and electric air source heat pumps that operate at the temperatures needed for integration with existing domestic radiator systems; innovative surface materials for buffering moisture, humidity and temperature; retrofit mechanical ventilation with heat recovery (MVHR) to manage ventilation and its associated heat loss. The technologies are being trialled in facilities that include the University of Nottingham E.ON 2016 House, a highly instrumented replica construction of a1930s dwelling. Alongside development and trialling, business case modelling of technologies is being conducted to establish mass roll-out strategies, as well as modelling to identify bespoke packages of measures for house refurbishment. This paper introduces Project CALEBRE, its content and scope, and reports some of its initial findings to highlight the challenges and constraints involved in the process of refurbishing the UK’s domestic stock.

Performance trends and heat transfer considerations in an ammonia–water resorption cycle

An experimental test facility was constructed to examine the potential of ammonia–water mixtures as the working fluid in high-temperature heat pumps. The nature of the working fluid necessitates an alternative design to the conventional vapour compression cycle. The addition of a solution circuit in parallel with the compressor leads to the resorption cycle. The composition of the working fluid can be altered by varying the flow ratio between the compression and solution pump circuits. Changes in the composition of the circulating fluid are accompanied by changes in the dryness fraction at the end of the heat transfer process in the desorber. Higher rates of heat transfer from the source to the working fluid were measured at higher concentrations of ammonia in the circulating fluid, though this was accompanied by lower overall flow rates of the circulating fluid. A 70/30 ammonia/water mass concentration is thought to be the optimum composition of the working fluid due to a combination of temperature glide and circulation ratio. Significant differences were observed in the overall heat transfer coefficient achieved in the two heat exchangers, which may restrict the range of likely applications. Copyright

R407c as an alternative to R22 in refrigeration systems

A refrigeration test facility was constructed to examine the performance of potential alternatives to R22. For the purpose of this paper, the performance of R407c (a zeotropic mixture of R134a, R125 and R32 in a 52% 25% 23% composition by mass) was compared to that of R22. It was found that the performance of R407c approached that of R22 at higher evaporator temperatures, but reductions in evaporator capacity and COPc were found with decreasing evaporator temperature. The effects on evaporator performance of a 32 cSt polyol ester compressor lubricating oil were also noted. It was seen that while R22 and this oil responded in typical fashion for a soluble refrigerant-oil pair (reduced capacity, increased evaporator pressure drop and fall-off in capacity at low superheats), R407c displayed a two-stage performance decline occurring over a greater range of superheat. The combined effect of differential solubility of the individual components of the mixture in conjunction with a more gradual solubility effect due to the presence of a temperature glide in the evaporator lead to a minor change in composition of the circulating refrigerant mixture resulting in a drop in evaporator capacity and COPc.