Integration of micro-cogenerations systems into existing buildings

1.Abstract

Because of the growing environmental awareness and the ratification of the Kyoto Protocol, combined heat and power receives again more attention as a way to contribute to the reduction of energy use and emissions.

Buildings account for 40 % of total energy consumption in the European Union. The sector is expanding, which is bound to increase its energy consumption.Nowadays, micro-cogeneration systems became a true and good solution to sustain the energy demand of industrial and even domestic buildings. Them are operating between 0 kWh - 50 kWh.

The district heating set up with a cogeneration system, concurs to attain energetic, economic and ambient benefits. It also provides to citizens a new service. The project strategy is based on the idea of supplying a portion of the necessary thermal power through a combustion alternative engine in cogeneration modality. It’s also interesting to modulate the load with auxiliary boilers fed by natural gas. This solution allows to save primary energy, create a centralization of the energy production, which contributes to the problem of polluting emissions, through the decentralization of the sources. The first step to assess the technical-economic feasibility of a district heating system, based on a cogeneration plant, is to underline and to characterize the energetic request of the basin of user.

2.Introduction

Cogeneration system is an ultimate technology, which can reduce energy cost and improve the efficiency. The CHP system can provide both type of energy, electrical and thermal. The benefits of this type of technology are worldwide know:

1. Increase efficiency of resources use: It is the most efficient technology in conversing the fuel power into electrical and thermal energy, reaching approximatively 30% energy savings, compared with separately production of those two types of energy.

2. Reduce of carbon foot-print: Cogeneration is a technology with reduce carbon emissions, having the potential of reducing, up to 30% of emissions for fossil fuels or 100% when you are using renewable sources of energy (biomass, biogas, etc.)

3. Compatible solution with solar panels: This type of systems, CHP, can produce electricity in a constant and flexible way, even in winter periods during the night, when the solar panel system that are in stand-by, or when them are producing energy intermittently.

4. Safety of energy supply: This system may be used like a safety supply power plant, in this way it is improved the energy supply system, in some cases additional components are needed and other operational standards.

5. Plug and play: CHP systems are make part from the few energy savings technology, which

can deliver heat at high temperature (more than 80 Celsius degree).

Usually, between 85 and 95 % from primary energy consumed by the cogeneration power plant, it is transformed in useful energy, which show us high efficiency, compared to other conventional systems.

For this technology to be feasible form economical point of view, it is necessary that the CHP system should be operated 14 hours per day or 5000 hours per year.

Figure 1.2. The schematic diagram of a cogeneration system (internal combustion engine)

So, if you want to adopt this solution to produce energy, to the detriment of conventional systems will go to these consequences:

- Low cost for electrical energy purchase (decrease the electrical energy purchase from grid, savings with regular taxes on distribution network and other electrical taxes)

- Extra incomes from incentives linked to cogeneration systems

- The opportunity to offer system services to electrical energy operator

- Improvement of carbon foot-print, reducing the environment pollution

Cogeneration or CHP units of 5-50 kWe are commercially available on the market. Most of these are based on reciprocating engine technology (gas engines), but also a few small appliances based on gas turbines can be found. They have a power-to-heat ratio of approximately 1:2, and the heat output of these units will generally then be within 15-120 kW. Internal combustion engines (ICE) dominate the market, but gas turbine is also a possible technology. The gas engines are spark ignited (Otto cycle) and operate either close to stoichiometric conditions allowing three-way catalysts to be used for emission reduction or are designed for lean-burn operation.

Often the system comprising a CHP unit also includes heat storage and additional boilers. The heat storage has several purposes, and the boilers are used for the heating demand when it exceeds the CHP heat output. To achieve the best electrical production efficiency and to benefit the most from the investment, the CHP units should (if possible) be operated at full load and for many hours per year. This means a sort of base load operation seen from a heat supply point of view. Ordinary and less costly boilers must then cover peak load heating periods or periods with low value for the electricity produced.

It is clear that the different technologies show a wide range of electric efficiency where the commercially available technologies, such as the Stirling engine unit and the combustion engine based units, are less efficient. Fuel cells for micro CHP purposes are still in the development and field test phase, except for Japan where they have been available for everybody since 2009. The energy balance for single-family houses in north-western Europe (UK, the Netherlands, Germany and Denmark) with an annual demand of approximately 5,000 kWh electricity and 17,000 kWh heating and hot water is roughly as follows: The micro CHP unit (ηel=30%) produces 5,000 kWh electricity, of which half is exported to the grid, and 10,000 kWh heat to be used for heating and hot water.