The objectives of this study are to present a model for biogas production systems to help achieve a more cost-effective system, and to analyse the conditions for connecting combined heat and power (CHP) plants to the biogas system. The European electricity market is assumed to be fully deregulated. The relation between connection of CHP. increased electricity and heat production, electricity prices, and electricity certificate trading is investigated. A cost-minimising linear programming model (MODEST) is used. MODEST has been applied to many energy systems, but this is the first time the model has been used for biogas production. The new model, which is the main result of this work, can be used for operational optimisation and evaluating economic consequences of future changes in the biogas system. The results from the case study and sensitivity analysis show that the model is reliable and can be used for strategic planning. The results show that implementation of a biogas-based CHP plant result in an electricity power production of approximately 39 GW h annually. Reduced system costs provide a profitability of 46 MSEK/year if electricity and heat prices increase by 100% and electricity certificate prices increase by 50%. CO2 emission reductions up to 32,000 ton/year can be achieved if generated electricity displaces coal-fired condensing power.

The way that owners of PV systems are handled today gives, in practice, installations of very small PV systems relative to what would be possible if all appropriately oriented roof and facade surfaces were fully exploited. This problem occurs because there is a surplus of PV electricity for the system owner, who receives a zero or low value in relation to the electricity purchases that are avoided. For single-family houses, this means that without net billing it is economically optimal to install only up to about 2-7 m2 of the approximately 60 m2 that are available on the roof of a single-family house. Other end-user types, such as multi-family buildings, agriculture and industry, also show low use of available surfaces. With the current system, the major part of the possible PV production on buildings is hindered. This electricity production does not exploit any new land and has a potential in Sweden of about 10-15 TWh, assuming that 25% of the roof and wall surfaces that have at least 70% of optimum solar radiation are exploited.

The effects of five different scenarios, without and with monthly or annual net billing for an electricity consumer who is also a PV electricity producer have been studied for ten different building types, including three single-family houses, two multi-family buildings and five other properties. The implications for four actors – the solar electricity producer, the grid owner, the electricity trader and the Swedish state – have been calculated. It is thus 200 different combinations that are reported. For each combination the outcome at any system size can also be seen in the reported figures.

The amount of saved electricity for the PV owner depends substantially on the time-horizon of the net billing period. Monthly net billing would drastically improve the utilization of roof areas, but still limits the utilization. Annual net billing gives a similar additional improvement. With annual net billing, the roofs of all the studied types of properties could be covered either entirely with solar cells or as much as needed to cover the annual needs of electricity. A net billing limit, for example 63A=43.5 kW=313 m2, would be a size delimiter for larger buildings.

Grid owners would be affected in the form of reduced revenues for the electricity transfer, reduced losses in the local grid and increased revenue from excess electricity which the PV owner donates to the grid.

For electricity traders increasing system size means that sales to the PV owner decrease in the same way as bought electricity is saved for the PV owner. The balance responsible actor (BA), which takes care of generated solar electricity, can usually make a profit due to the price profile. This could also be the grid owner, or the BA designated by the grid owner, or an electricity trader chosen by the system owner depending on how the net billing is handled. If the same electricity trader is affected by the reduction in electricity sales and earnings due to the price profile, this will be favourable for the electricity trader.

Looking at tax from PV installations, net billing has the same economic effect as if the PV owner had made an energy efficiency measure. The calculations have not taken into account the state's tax revenue of the investment, which today is higher than the loss of revenue for energy tax and VAT.

For the further development of the PV market in Sweden it is of utmost importance to make it possible, as soon as possible, for PV system owners to get a reasonable compensation for their excess electricity. Net billing would be an easy way to solve this problem. The most practical and easiest way to achieve net billing would be if the grid owner could send a net value to the electricity trader. The period for net billing should be longer than one month if all available roof and wall areas are to be optimally utilized.

This paper presents some of the results of a power system analysis for Chile. The two major Chilean electric systems are roughly modelled and optimized using a linear programming method with the option to integrate renewable energy sources like wind power, solar power, mini-hydropower and biomass-fired power and also "municipal waste". A total of four different scenarios are outlined: reference system, new production units, gas and coal price variations and a policy measure to encourage power production based on renewable energy. The objective of the scenarios was to illustrate under what conditions integration of the different energy sources in the existing production system is possible. The study shows that even under current conditions, mini-hydro and waste to energy plants are economically viable. Wind power might be interesting alternatives if policy instrument measures are applied. On the other hand, it is hard for the other energy sources to enter the system even when higher price levels of gas and coal are applied. The system is more sensitive to coal price increases than to gas price increases and this mainly encourages CO2 emission reduction.

The potential for combined heat and power (CHP) generation in Stockholm is large and a total heat demand of about 10 TWh/year can be met in a renewed large district heating system. This model of the Stockholm district heating system shows that CHP generation can increase from 8% in 2004 to 15.5% of the total electricity generation in Sweden. Increased electricity costs in recent years have awakened an interest to invest in new electricity generation. Since renewable alternatives are favoured by green certificates, bio-fuelled CHP is most profitable at low electricity prices. Since heat demand in the district heating network sets the limit for possible electricity generation, a CHP alternative with a high electricity to heat ratio will be more profitable at when electricity prices are high. The efficient energy use in CHP has the potential to contribute to reductions in carbon dioxide emissions in Europe, when they are required and the European electricity market is working perfectly. The potential in Stockholm exceeds Sweden's undertakings under the Kyoto protocol and national reduction goals.

The result presented in this paper shows that the Volvo plant can decrease its electricity use by 44% by making the use of electricity more efficient and converting from oil and electricity to district heating for hot tap-water, space heating and cooling. The increased demand of district heating makes investing in a new planned CHP and cooperation between the Volvo plant and the local energy utility production cost fall by 46% at current unit electricity price and by 64% when calculating with a European unit electricity price and investment in an optimised CHP system instead of the planned plant. The study furthermore shows that the global emissions of the greenhouse gas carbon-dioxide will be reduced by 350% a year if the two energy-supply measures are taken and the electricity unit prices are at a European level.

In 2004 Sweden will become part of a common European electricity market. This implies that the price of electricity in Swedish will adapt to a higher European electricity price due to the increase in cross-border trading. Swedish plant is characterized as more electricity-intensive than plant on the European continent, and this, in combination with a higher European electricity price will lead to a precarious scenario.

This paper studies the energy use of 11 plants in the municipality of Oskarshamn in Sweden. The aim is to show how these plants can reduce their electricity use to adapt to a European level. We have found that the plants could reduce their use of electricity by 48% and their use of energy by 40%. In a European perspective, where coal-condensing power is assumed to be the marginal production that alters as the electricity demand changes, the decrease in the use of electricity in this study leads to a reduction in global emissions of carbon dioxide of 69,000 tonne a year.

Electricity generated in Sweden emits very low emissions of carbon dioxide and have thus consequently very low external cost. The freed capacity in Sweden could therefore replace electricity generated with higher external cost and as a result lower the total external cost in Europe. The emissions from the saved electricity could also be valuable within the EU emissions trading scheme, if the emissions calculation is done assuming the marginal electricity is fossil fuel based.

A study is presented where factorial design is used to find how some selected economic and technical factors affect the profitability of an investment in a combined heat and power plant. The study is performed on a Swedish district heating system. The minimal cost for supplying the demanded heat is calculated with a developed energy system optimisation model, MODEST. The effects on the resulting parameters, such as system cost and optimal size of steam cycle, are calculated from a series of experiments performed using high and low levels of the most relevant factors. The conclusion of the study is that both the main factors and the interactions between them have to be analysed to establish an accurate ranking of the technical and economic factors. (C) 2000 Elsevier Science Ltd. All rights reserved.

In the Swedish electricity system there is a great potential for increasing the cost efficiency of electricity use. Today economic incentives, offered for instance by existing electricity tariffs, are too weak to improve the use of the system. On the Swedish electricity market, there are at least three different participants, the power producer, the distributor and the customer. Today these participants act separately owing to low awareness of the costs for electricity over the year and the day. If the participants are aware of the real electricity costs, cost-effective incentives for cooperation will arise. When participants cooperate, the introduction of end-use measures will reduce system costs for those participants that are involved in cooperation. We present a system analysis for cooperation between distributor and customers. We also present results from a project, where behaviours of an existing distributor and existing customers have been analysed. The results show that there exist cost-effective incentives for cooperation when end-use measures are introduced.

If actors on the electricity market cooperate when end-use measures are introduced the energy-system, cost will be reduced considerably. The marginal cost for electricity for the energy system of the actors will show cost effective incentives for introducing end-use measures. We present a system analysis for cooperation between distributors and customers. We also present results from a project where an existing distributor and eleven existing customers within a municipal energy system have been analysed. The customers are various industries, a hospital, an ice hockey arena, a harbour, a water-works, a warehouse, and a radio tower. The results show that the customers have in different end-use measures a power reduction capacity of maximum 8642 kW. With electricity costs of 1994 this corresponds to a reduction in the energy system cost of 2,852,000 SEK for one year. The results also show that for the distributor`s load curve of 1994, the full power reduction capacity can not be used since the peak loads of the five winter months are not so large and distinct. In that case the energy system cost can be reduced by 1,909,000 SEK, which is 67% of the maximum cost reduction. The end-use measures that are cost effective in this municipal energy system are load management and electricity generation in reserve power plants. We have also studied the profitability for introducing bivalent heating systems based on oil and electricity for heat loads that originally are based on oil. However, with existing electricity and oil costs there are no incentives for increasing the electricity use during non-peak load periods with bivalent heating systems.

Mathematical models are extensively used in energy analysis and have increased in scope as better and faster computers have become available. With complicated systems, it is difficult to predict accurate results if doubtful input data are changed. Traditionally, sensitivity analysis with a change of one or more of the parameters is used. If the influence of a change is very small, the first result is believed to be accurate. Problems may arise when sensitivity analysis is applied to a vast amount of data. The aim of this paper is to examine whether the calculation effort can be decreased by using factorial design. Our model, called Opera (Optimal Energy Retrofit Advisory), is used to find the optimal retrofit strategy for a multi-family building. The optimal solution is characterised by the lowest possible life-cycle cost. Three parameters have been studied here: length of the optimisation period, real interest rate and existing U-value for an attic floor. The first two parameters are found to influence the life-cycle cost significantly, while the last is of minor importance for this cost. We also show that factorial analysis must be used with great care because the method does not reflect the complete situation.

In a national electricity system there often exists a great potential for increasing the cost-efficiency of the electricity use. However, if the economic incentives for improving the use of the system are too weak, it is most likely that this potential will not be utilised. If electricity tariffs reflect real electricity costs, over the year and the day, cost-effective incentives will arise for introducing energy system measures that will reduce the energy system cost considerably. This paper presents two energy system analyses of an existing Swedish municipal energy system. The analyses are carried out with a simulation model for electricity use in industrial energy systems, and an optimisation model that is based on linear programming.<>

This paper will describe an energy system analysis of an existing Swedish municipality of 90 000 inhabitants. The analysis, which is performed by using an optimization model will show what energy system measures that should be introduced to minimize the total energy system cost. In the existing municipality a local utility distributes heat, for the district heating system, and electricity. The heat is generated by the utility and the electricity is purchased from a large power producer.

In a Combined Heat and Power (CHP) network, it is sometimes optimal to install a device for storing heat from one period of time to another. Several possibilities exist. If the electricity demand is high, while at the same time the district heating load is too small to take care of the heat from the CHP plant, it could be optimal to store heat from peak periods and discharge the storage under off-peak. It might also be optimal to store heat during off-peak and use it under the district heating peak load. The storage is then used for decreasing either the district heating demand or for decreasing the electricity load used for space heating. The paper shows how a mixed integer program is developed for use in the optimization process. As a case study, the CHP system of Malmö, Sweden, is used. Further, a sensitivity analysis is elaborated in order to show how the optimal solution will vary due to changes in certain input data.

New office buildings in Sweden are thoroughly insulated due to the Swedish building code. This code, however, does not consider the type of activity occurring in the building. This means that the heating equipment is designed as if no activity at all is going on. In modern offices there is a lot of equipment installed which uses electricity. This electricity is converted into heat which can be utilized for heating the premises, mostly in a direct way but also by the use of exhaust-air heat-pumps or heat exchangers. This paper deals with a modern office building plus office hotel complex located in Linköping, Sweden, about 200 km south of Stockholm. The tenants deal with the design of hard- and software for computers. The lighting and computers in the building use electricity which converts into heat. In this paper, it is shown that this electricity is all that is needed during normal conditions, i.e. when people work in the building. The building is also equipped with a district-heating system, which is designed as if no activity goes on in the building, so subsequently the heating equipment is larger than it need be. In this special case, it might have been better to install an electric heating device for hot-water heating and very cold winter conditions, instead of using district heating. This is so even if district heat is about half the unit price compared with that due to the dissipation of electricity. At present, when district heating is used, no measures for saving heat can be profitable due to the low district-heating price. The fact is that the tenants complain of too much heat instead of too little: the prevailing indoor temperature was about 24° C in January 1990 even though 20° C would have been sufficient. There is subsequently a need for a properly working regulation system. The one currently in use is designed to a modern standard, but is not able to maintain temperatures at a modest level.

This paper shows how to simulate a CHP network (CHP = Combined Heat and Power) using the method of linear programming. This method makes it possible to optimize the mathematical model and subsequently find the very best combination of electricity production, electricity purchase and heat production in a district heating system. The optimal solution in the model is characterized by the lowest possible operating cost for year. The paper shows the design of the mathermatical model and furthermore a case study is presented using the district heating net in Malmö, Sweden, as an example.

This paper deals with the interaction between different types of building energy retrofits. The means for finding this interaction has been via the OPERA model, which is used for energy retrofit optimization. The solution is an optimum when the total life-cycle cost, LCC, for the building, i.e. the sum of the building, maintenance and operating costs, is minimized. The model finds the candidates for the optimal strategy by calculating the total LCC for one retrofit after another, i.e., an incremental method is used. All the measures are implemented with respect to the building and the resulting LCC is calculated. Usually, the LCC for this combination is higher than the incremental LCC, i.e. the incremental way of calculation overestimates the savings. However, when window retrofits are considered, the opposite might happen due to the use of shading factors. These factors indicate the decrease in solar radiation through a window when an ordinary one is replaced by a window with enhanced thermal performance. The paper also shows that the interaction between the different measures usually can be neglected, as long as optimal retrofits are introduced.

In accordance with a public referendum held in 1980, Sweden will phase out nuclear power completely by 2010. One way to compensate for an immediate, appreciable scarcity of electric power is to construct new fossil-fuel power stations. Another is to reduce the burden on electric power by converting some end-user facilities to operate on natural gas (NG) imported from Denmark through a new pipeline to southern Sweden. We show how an optimal solution can be found for NG operation of a system incorporating an NG boiler and an electric heat pump. Electricity is priced by a time-of-use tariff (TOU) requiring a discrete optimization method. The optimal solution is characterized by the lowest life cycle cost (LCC) for the building as an energy system.

In Sweden, Combined generation of Heat and Power (CHP) is in common practice. Different fuels are burnt in a boiler and the steam is used for generating electricity. The heat that has to be transferred from the condenser in the plant is used in the district heating grid. This grid is thus used as a cooling facility necessary for electricity production. However, energy conservation the Swedish building stock is also encouraged, and if this is utilized in district heated buildings it results in fewer possibilities for electricity production. This might be a major drawback when nuclear power is abolished, as is the result from a consensus some years ago. This paper deals with the question of whether it is better to conserve both heat and electricity, to save only one of the energy forms or if it is cheaper to produce more energy, instead of saving. A case study is presented dealing with Malmö, in the South of Sweden, and it is shown that energy conservation in district heated buildings cannot yield profitability: neither can conservation in the electricity grid, even if it gets closer to profitable savings. It is assumed that the total cost of heating, insulation and electricity is paid by the society and the minimum point for this cost will characterize the best solution.

Time-of-use tariffs, which reflect the cost of producing one extra unit of electricity, will be more common in the future. In Sweden the electricity unit price will be high during the winter and cheaper during the summer. A bivalent heating system, where an oil-fired boiler takes care of the peak load, when the electricity price is high, and a heat pump the base load, may decrease the cost of space heating substantially. However, insulation retrofits are also likely to reduce the peak space-heating load in a building. This paper shows how a bivalent heating system can be optimized while also considering the insulation measures. The optimization is elaborated by the use of a mixed integer programming model and the result is compared with a derivative optimization method used in the OPERA (optimal energy retrofit advisory) model. Both models use the life-cycle cost (LCC) as a ranking criterion, i.e. when the lowest LCC for the building is achieved, no better retrofit combination exists for the remaining life of the building.

When a building is to be renovated it is important to implement the optimal retrofit combination. If this strategy is neglected it might not be profitable to change the building in order to improve it as an energy system. This paper deals with energy retrofits and how the strategy can be optimized considering one specific building. The best solution is found when the life-cycle cost for the building is minimal, and building envelope, ventilation and heating system retrofits are combined.

In order to solve the problem, a mathematical model, OPtimal Energy Retrofit Advisory (OPERA), has been developed. Energy balance calculations are used in which the free energy from solar radiation and from appliances is taken into proper account. The interaction between different retrofits is emphasized. Provided that the optimal solution is implemented, the retrofits in the combination will have a minor interaction which, for most cases, could be neglected. This will also imply that the order of implementation is of no, or minor, importance. A case study for a real building sited in Malmö, Sweden, and a sensitivity analysis for some critical input parameters are discussed.

The cost for producing energy differs a lot due to the load coupled to the distribution grid. In Sweden the load has its maximum during the winter because of the climate. The cost for producing one extra unit of energy is then about 0.50 SEK kWh⁻¹ (1 US$ = 6 SEK), while during summer the cost can be ten times lower. In order to encourage the consumers to save energy during the winter when the cost is high, it may be important to introduce a time-of-use tariff, which reflects the cost for producing the energy. Such a rate is present in Malmö, Sweden. when retrofitting buildings it is of course important to consider the applicable rate for energy, in order to decide the optimal retrofit strategy. In the time-of-use rate the peak load is expensive and a heating system that will use less of the peak energy becomes very desirable. A bivalent heating system, where the base load is provided by a heat pump and an oil boiler takes care of the building peak load can sometimes be found to be the best solution. In this paper two different methods are used for the optimization of such a bivalent heating system. One method uses derivative considerations, the OPERA model, while the other uses linear programming.

Using life-cycle costing (LCC) gives us a means to find the best retrofit strategy for an apartment block. This method also shows us how important it is to consider the whole existing building as an energy system. If the best heating system is put into the house almost every shield retrofit is unprofitable. Having heating systems, with high variable costs combined with exhaust ventilation air pumps, sometimes makes it unprofitable to caulk the windows and doors.

This article also shows the importance of using the accurate prices for the energy. Short-range marginal costs (SMRC) gives different retrofit strategies than normal tariffs used today. This also means that the retrofits do not correspond to the optimal use of the total national energy system and already scarce resources are used unnecessarily.

When renovating existing multi-family buildings it is very important to implement the best retrofit strategy possible in order to minimize the remaining life-cycle cost for the building. If the building is heated with district heating this strategy of course changes due to the energy rate used by the utility. It is also very important for the utility that the consumer is encouraged to save energy when there is a need for it, i.e. during peak load conditions. Our paper shows that an accurate cost differential rate provides all these facilities.