How to choose a CHP unit, buy a cogeneration unit

A cogeneration unit has a service life of 20–25 years. The parameters set when the equipment is selected determine the economics of the project for that entire period: output, operating regime, heat utilisation scheme and gas supply conditions. Below are five questions to answer before requesting a quotation. How to choose a CHP unit, buy a cogeneration unit

1. What is the plant’s base electrical load How to choose a CHP unit, buy a cogeneration unit

CHP output is selected against the base load — the consumption level the plant runs at most of the time. Peak loads (simultaneous start-up of lines, compressors, refrigeration equipment) are covered from the grid. In this regime the unit runs at rated output with the highest fuel utilisation factor. A unit sized for peak load runs underloaded most of the time, and the payback period increases.

Selection across the Dieselmash range:

■  base load 400–500 kW — DvGA-500;

■  550–620 kW — DvGA-630;

■  above 620 kW — a cascade of two units or DvGA-750;

■  small production sites, biogas farms, greenhouses up to 1 ha — DvGA-315.

A cascade of two DvGA-500 units delivers 1 MW and allows output to be regulated: one unit runs at base load, the second comes online as demand rises.

Determining the base load requires electricity consumption data for the past 12 months, ideally hourly. Summer and winter profiles of an industrial plant can differ by a factor of three to four, so an annual average is not suitable for sizing.

2. How many hours a year will the unit run

Savings from a CHP unit accrue every operating hour as the difference between the cost of self-generated and purchased energy. The payback period depends directly on annual running hours.

■  4,000 h/year and above where gas is available — CHP. For most industrial sites in continuous operation, payback on natural gas is 1.5–2.5 years.

■  12–16 hours a day, seasonal heat demand — payback on natural gas is typically 3–4 years.

■  Standby duty of 500–1,000 h/year — a diesel generator: at these running hours a CHP unit does not pay back.

■  Calculated payback above 5 years — grounds to consider other solutions: a standby diesel generator, solar panels with storage, or a hybrid scheme.

Feasibility study of 01.07.2026 for DvGA-500: 8,000 h/year, gas UAH 30/Nm³, electricity UAH 12/kWh. Turnkey project cost — UAH 20 million, annual economic effect — UAH 11.7 million, payback period — 1.7 years.

A comparison of CHP units and diesel generators at different running hours is given in the article “500 kW CHP unit or 500 kW diesel generator”.

3. How the thermal energy will be used

The CHP unit recovers heat from the exhaust gases (400–500 °C) and the engine coolant (90 °C) and delivers it to heating, hot water supply or process needs. Recovered thermal output: DvGA-500 — about 720 kW, DvGA-630 — about 900 kW.

The scale of the saving on heat depends on how evenly heat is consumed through the year: the closer the heat demand profile is to the unit’s operating regime, the greater the effect. In the feasibility study of 01.07.2026 the displaced heat amounts to 2,228 Gcal per year at UAH 2,000/Gcal — about UAH 4.5 million of annual saving; this volume is calculated for the heating season. With year-round demand the effect is higher, with partial demand it is lower.

Where there is no heat consumer, the unit operates with excess heat rejected, and payback is calculated on the electrical side alone and increases.

Heat is used for space heating and hot water supply, process water heating, product drying, digester substrate heating and greenhouse heating. Year-round heat demand is typical of dairy and meat-processing plants, greenhouses, municipal district heating utilities, agri-industrial enterprises with processing, and hotel and resort complexes. Where heat is required only during the heating season, this is reflected in the feasibility study.

Where a boiler house already exists, integrating the CHP unit into the existing infrastructure is the preferred route: thermal circuit design is shortened by 3–4 weeks and CAPEX is reduced.

4. What are the gas parameters at the connection point

■  Pressure. Dieselmash CHP units operate on gas at 0.1–4 bar. Below 0.1 bar this is accounted for in the selection of gas equipment included in the scope of supply. The pressure value is stated in the technical questionnaire (terms of reference).

■  Gas quality. The calorific value of natural gas varies between regions of Ukraine and determines consumption. Rated consumption of the DvGA-500 is 133 Nm³/h at a calorific value of 8,600 kcal/Nm³. The actual calorific value at the connection point is stated in the technical questionnaire and built into the feasibility study.

■  Contract with the gas distribution operator. Gas pipeline design and approvals are typically the longest stage of the project. The absence of a valid contract, or disputes over the balance-sheet boundary, are resolved in parallel with equipment selection.

Where the site has its own feedstock (manure, silage, processing waste), alternative fuel is considered: biogas, producer gas or pyrolysis gas. A hybrid scheme means commissioning the CHP unit on natural gas and upgrading it later for alternative fuel while retaining the ability to run on natural gas. See the PE Sigma case study and the winter readiness checklist.

5. What a turnkey supply includes and how service is organised

“Turnkey” is interpreted differently by different suppliers: in some cases it covers only the equipment on the customer’s foundation, without the gas pipeline, grid connection or design work. Underestimating these items is a common reason for project budgets being exceeded by 20–40%. A turnkey supply means a unit ready for industrial operation.

The scope of supply is divided into a standard and an optional part. The standard part covers the equipment and work without which the unit cannot be placed in industrial operation. The optional part depends on the conditions of the specific site: the design of the unit, the scheme of connection to utilities, and noise and siting requirements. Both parts are agreed at the technical questionnaire stage and recorded in the contract, which allows the full project cost to be seen before work begins.

Before signing, confirm that the scope of supply includes commissioning with load testing, training of operating personnel, and documentation and data sheets for all components, and that responsibility for connection to the gas pipeline and the electrical network is clearly allocated between the parties.

The standard warranty on Dieselmash units is 1–2 years. After the warranty period, servicing is performed under a service contract with the manufacturer or by the customer’s own trained personnel. Maintenance schedule: intermediate overhaul after 40,000 engine hours, major overhaul after 80,000, total engine life of the 6ГЧН25/34 and 8ГЧН25/34 — 240,000 engine hours. The unit is shut down for repairs, so a standby source or switchover to the grid is provided for in advance.

Input data for the feasibility study

■  electrical load profile — hourly, daily or monthly for the past year;

■  thermal load profile — hot water, steam, heating, by season;

■  plant operating regime — hours per day and per year;

■  electricity and gas bills for the past 12 months;

■  connection specifications — gas pipeline capacity, capacity of the grid connection point;

■  area available for the unit: a containerised DvGA-500 module needs about 25–30 m², the DvGA-630 about 30–35 m².

Where some of this data is unavailable, Dieselmash engineers calculate indicative values based on the type of production. Technical specifications of the models are given in the article “DvGA-500 and DvGA-630: technical specifications”.

Payback calculation for your site

State your load, operating regime, gas availability and heat demand. The feasibility study is prepared within 3–5 working days, free of charge.